Early metaphorical use of 'a smear' and 'political smear'
The earliest match that I find of "a smear" used in the sense of a besmirching of [one's] reputation appears in an untitled item in the Terre Haute [Indiana] Weekly Gazette (January 9, 1879):
Hiram Grant's reception in Ireland can hardly be called a success. It is about time he ceased traveling in part as Government expense, and he has, perhaps, traveled abroad as much as it is proper on the credit and reputation of this country, his connection with which, as President, is a smear of dirt in the recollection of all honest men.
Here, the "smear of dirt" is in the recollection of honest men, but it is on the connection of Hiram [better known as "Ulysses S."] Grant with the country and—by extension—its credit and reputation. Evidently, the newspaper is alluding to various scandals that plagued Grant's administration, which ended in 1877. The sense is not of an inaccurate characterization or imputation, but of a point of justified embarrassment or disgrace.
A somewhat similar instance in an Australian newspaper appears in an untitled item in the [Adelaide] South Australian Advertiser (August 17, 1885):
To Lord Granville's peremptory demand that Russia should withdraw from the places which she had illegally seized the insolent reply was sent that Russia could cot retire, as it would injure her prestige. Therefore the honor of England had to be trampled upon, and the territory of her ally violated without redress, in order that Russian prestige might escape a smear. Much as the British character resents anything wearing the semblance of bluster in the utterances of public men, or proceedings calculated to give needless offence to other nations, the manly determination of Lord Salisbury to act in complete indifference to European combinations will be generally applauded.
Again, the smear is closer in sense to a reputational stain or blemish than to a false claim about one's conduct.
There is also this interesting presentation of an editorial spat between the Vermont Phoenix and the Brattleboro Reformer, reported in the Bennington [Vermont] Evening Banner (July 17, 1906):
We do not remember having seen such a smear in the editorial columns of our neighbor since—well, in a long time. What is there to prevent a blacksmith, livery stable employee, fish dealer, ex-bartender, lunch room proprietor, shoe repairer, cigar salesman, grain sacker, hack driver, or barber from being as good a citizen as a "prominent or professional man with large interests"? If the Phoenix considers such tactics of assistance to the Proctor cause its judgment is certainly warped.—Brattleboro Reformer.
It's unclear whether the "smear" in this case refers to a perceived attempt by the Vermont Phoenix to insult to the reputations of the tradesman delegates from Brattleboro or to a general waste of ink by the Phoenix on a muddled thesis.
A related usage is the compound of "a blot and a smear," a metaphor taken from writing with pen and ink. Two relevant earlier instances of this expression occur, first, in an untitled item in the [Stanford, Kentucky] Interior Journal (October 24, 1879):
The split from the Democratic party by faction led by John Kelly was made to defeat the Democratic ticket this time, but reports from that state [New York] go to prove that it will be unsuccessful and that Gov. Robinson will be his own successor in office. The Kelley meanness has fired the true blue Democrats to action, and there will be nothing left of Kelly and his Tammany crowd after the election but a blot and a smear.
The instance of "a blot and a smear" here seems to refer to an unattractive residue of something other wise obliterated.
But something quite different is suggested in "Easy Divorces Not for Golden State" in the Sacramento [California] Union (March 27, 1911):
Senator Bell of Pasadena was opposed to the policy of California becoming a Mecca, for the mis-married. "Who would go to Reno [Nevada] when they could come to California?" inquired the senator, and continued: "If we should pass this bill every aspiring divorcee in the country would come flocking to California. Some person would establish a colony, such a colony as the world has never known, for the attractions of the state would bring the merry martyrs of marriage from all parts of the nation, causing a blot and a smear on the reputation of the state."
Here "a blot and a smear" refers to an unsightly marring of something otherwise pure and honorable, much as in the "a smear of dirt" example from January 1879.
The earliest match for "political smear" that I could find is in the subtitle to "Republicans to Probe Scandal" in the Champaign [Illinois] Daily News (January 18, 1916):
Political "Smear" to Be Investigated Before Action Is Taken on Preparedness
But since the situation involves Republican congressmen investigating possible improprieties by a Democratic administration, the "smear" again seems to refer to a stain on the honor of the national defense program, not to a groundless effort to discredit that program.
Another longer expression of interest, "a smear gang," first appears in "Hurley Attacks Liberty League," in the Madera [California] Tribune (August 290, 1934):
Former Secretary of War Patrick J, Hurley today criticized sharply the newly formed American liberty league and charged it was financed by the same group that led "the Smear-Hoover campaign."
"I have never joined a Smear gang." said former President Hoover’s cabinet member.
Hurley did not identify the "Smear Hoover" leaders to whom he referred. It was recalled, however, that Jouett Shouse, President of the league, was executive head of the Democratic national committee during the Hoover administration when Republican leaders complained that the committee’s publicity campaign was unfair.
And at long last, a relevant instance of the unadorned phrase "a smear" appears in "Washington Letter," in the Sausalito [California] News (October 2, 1936):
The campaigners have no way of measuring the effects of a smear which was splashed around after [Al] Smith's speech at the Liberty League dinner when he was allied with the DuPonts and big money men.
Very early metaphorical use of 'to smear'
Instances in which the verb form of smear in the sense of libelous falsehoods or more general political mudslinging appear in various U.S. newspapers at least as early as the 1830s.
From "Things in General" in the [Terre Haute, Indiana] Wabash Courier (July 23, 1835):
These scullions, who are themselves up to the elbows in grease are anxious to smear every opponent who comes within their reach. Noah.
This very brief item expresses the metaphor in terms of smearing grease that is actually on oneself onto someone who was not previously so befouled.
From "Speech of Mr. Powell, of Loudoun, on the Expunging Resolutions, on the 22d of January, 1836" in the [Leesburg, Virginia] Genius of Liberty (February 6, 1836) comes this very modern use of smear:
Sir, Mr. Leigh must be forced to give way before the patriotic efforts of this great Democratic Republican Party of Virginia. How, sir, and for what? By an instruction on their part for Mr. Leigh to do that which they know he cannot consistently do, or to submit to an alternative that they have no right to present to him. They require at his hands, either directly or by implication, to expunge, to obliterate, to smear with the miserable filth of party politics, that journal of the Senate which the Constitution of the United States has declared shall be kept as a record of their proceedings, or to follow the example of his diplomatic predecessor, and throw himself back insultingly upon the people.
From a letter to the editor of the same newspaper (July 16, 1836):
Every event subsequent to the removal having any relation to the matter, fully establishes that the public interest did not require the act, and that it has received much inconvenience and detriment from a measure, the sole intention of which was to castigate the bank for its lack of Executive loyalty—and because the Senate did not approve the course of an Executive, who had dared to effect his purpose by an infraction of the Constitution, it is proposed to smear upon its journal the indelible stain of Jacksonism. A striking evidence that his unjust and arbitrary course towards the bank grew out of his inability to direct its operations, is to be found in his continued wrath and persecuting opposition to it as a state institution.
Here "Jacksonism" is the stain to be smeared, and Andrew Jackson is the smearer.
From "Abuse," in the [Cincinnati, Ohio] Catholic Telegraph and Advocate (April 3, 1852):
During the past two weeks, the Catholics have been abused most thoroughly, collectively and individually, theologically and morally, historically, politically, socially and abominably. ... When we find such extravagant and incessant assaults, mixed with so much intemperate invective, are we not right in attributing it to some uneasiness of conscience, some inherent suspicion that the contradictions of Protestantism are not very satisfactory, and that all this anger which is displayed in the sectarian pulpit and the twaddling press, is only an attempt to lull the soul and comfort it with an imaginary security? It is an easy thing to brag of your beauty when compared with another's, whose face you have taken the precaution to smear with filth.
And from an untitled item in the Dayton [Ohio] Daily Empire (July 31, 1860):
The room was pretty well filled with admirers and worshippers of the man [Cassius M. Clay, "the noted Kentucky agitator" for abolition], and every body in the vicinity seemed anxious to get a sight of him. Among those who came to see and hear him, were many persons who a few years ago would have denounced his principles, and furnished eggs to smear him! "Things is workin.'" If the Abolitionists keep on in the same ratio, it will not be lung until they will have every conservative man in the Republican ranks converted to the true faith, as taught by Clay, Giddings, Brinkerhoff & Co.
The egg smearing in this instance may have been intended literally.
An interesting early counterintuitive instance of smear appears in a letter to the editor of the [Launcestown, Tasmania] Cornwall Chronicle (October 23, 1844), where the smearing is more in the nature of a whitewashing or perhaps a lipsticking of a pig:
SIR,—What a pity it is, Mr. Editor, that self-praise is no honor, for if it were, the committee of the self-styled Mechanics' Institute of this town would be covered with immortal laurels. But is it not false and foolish in the lying Bolter's Rag to smear them over with such fulsome daubing. Does he not know that he is only caricaturing them while he wishes to exalt them.
The daubing is revolting (to the letter writer) but the self-dauber imagines that it makes him more lovely.
It is at least possible that "smearing" in the befouling sense is associated with the contemporaneous U.S. vigilante punishment of tarring and feathering, as described in this item from the [Springfield] Illinois State Journal (October 26, 1854) about such an outrage committed in Ellsworth, Maine:
The ruffians, as soon as they were rid of the Sheriff, came together again and proceeded down the Mount Desert road to a ship yard. Here they took 'the pantaloons also from Mr. Bapst's person, rendering him entirely naked. Before taking off his pantaloons, they lifted him among them, and he perceived one thrusting his hand in his pocket and laying hold of his wallet, in which was something more than $50. He besought that one not to add robbery to his other violence, but the person took his wallet, nor has Mr. Bapst seen it since. " His watch wag taken from him at the time that they first stripped him. Then they proceeded to smear him with tar from head to foot, and afterward covered him with feathers not without, however, in the meantime making some disgusting personal assaults, and using various degrees of foul language.
Relevant instances from the 1870s and after of 'to smear' and 'smear campaign'
After several decades of infrequent usage, a number of examples of "to smear" in the sense of "to assault a public reputation" appear during the 1870s. From "The Military Ring," in Harper's Weekly (March 23, 1872):
Those who in the investigation of frauds in administration seem much more anxious to smear the President than to punish guilty agents ought to consider whether by so clear an exhibition of personal animosity they do not harm the cause of simple, honest reform ...
From an untitled item in the Weekly [Troy] Kansas Chief (October 24, 1872):
Mart. Bowers takes exceptions to our calling him a carpet-bagger. To tell the truth, we would not have thought of it, had not the idea been suggested to us, not over three weeks ago, by the very men who are now upholding his course—the men who used to smear him all over with foul gobs of nastiness, and whom he used to smear with ditto.
This same newspaper, in "A Liar Is Known by the Lies He Tells" (September 26, 1872), uses a boil as the thing metaphorically being smeared:
But the boil has at length come to a head; they have pricked it, and the nasty matter is let out. The worst it can now do, is attempt to smear and defile the garments of those who have squeezed it out of the party.
Less grotesque is this example from the Donaldsville [Louisiana] Chief (June 26, 1875):
We do not hesitate to express the belief that among the journalists of New Orleans the Republican is almost universally considered the ablest and best edited paper in the State. In our estimation it stands at the head of the Southern press, and we are sorry to see a co-worker in the Republican cause so eager to outstrip the virulent organs of Democracy in attempting to smear the fairly won laurels of a natural ally with the slime of vituperation. We have enough to contend against without rending each other, brethren.
And this similar example from "It Didn't Stick," in the Findlay [Ohio] Jeffersonian (December 3, 1875):
The ringsters who threw mud at Secretary Bristow, failed in their attempt to smear him over with the slime that covers their own carcasses. It didn't stick. Mr. Bristow speaks out in tones and in language that is impossible to be misunderstood. He means business in his war on the "Crooked Whiskyists," and don't propose to let "any guilty man escape."
And this one from "The Constitutional Convention Again," in the [Alexandria] Louisiana Democrat (August 22, 1877):
Our article upon this subject of week before last has given umbrage to The Daily City Item, and necessitates our referring to the question again. We accord the Item the utmost freedom of opinion in the matter, and the perfect right to differ with us—nay, even to criticise our opinions as expressed in the article referred to; but it ha gone outside of the range of legitimate criticism, and seen fit to impugn our motives in opposing the call of a Convention to frame a new Constitution entire. We have no desire or inclination to enter into a war of words with the Item, as we fear that its recent experience with the Times has "got its hand in" and that it would have the advantage of us in that species of choice literature, that seeks rather to smear an opponent with slime than to answer arguments fairly stated.
The metaphor in these three examples is explicitly smearing with slime, a strikingly popular understanding of what smearing involved during the 1870s.
In much the same vein, though some 55 years later, from an untitled item in the Craig [Colorado] Empire Courier (November 2, 1932):
Alva Adams was nominated by the Democrats through the use at the last minute of a "Smear Barnett with Oil" campaign for which Mr. Adams later denied responsibility. He is now attempting to "smear Schuyler" with the same tar. An analysis of the oil charges against Schuyler indicates that he was retained as attorney for Blackmer and others implicated in the oil deals. If that constitute guilt then we haven't an experienced attorney in Colorado who is not implicated in some questionable practice.
This is the earliest example of the concept of a "smear ... campaign" that I've been able to find. We've already seen an instance of "smear Hoover campaign" from 1934.
The more compact phrase "smear campaign" appears in "Our Political Pessimist," in the [Healdsburg, California] Sotoyome Scimitar (July 29, 1937):
Next year promises to bring a comparatively new development—the "smear campaign", given its first tryout last year by the Donihue assembly investigating committee. Both sides are investigating now—Republicans and Democrats and fractions within both parties—and wire-tapping has replaced wire-pulling as the favorite sport of politics.
The verb smear in the sense of "falsely or dishonestly besmirch"—as opposed to "dishonor, whether justly or not"—has been around in U.S. English since at least the 1830s, although its use in the popular press seems to have been rather sporadic until the 1870s. Use of smear as a noun in a similar sense involving strategic recourse to unsubstantiated accusations appears to be far more recent than use of smear as a noun in the sense of a stain on one's honor that might or might not be justified.
As for what material the metaphorical smear originally consisted of, candidates through the years have included grease, filth, the stain of Jacksonism, a boil, vile gobs of nastiness, slime, dirt, eggs, mud, oil, tar, and ink. Each of these substances has with its own constellation of allusive reference points and thus its own metaphorical strengths as a smearing material. The very range and variety of potentially suitable substances may have broadened the appeal of the expression.