Instances in print of the phrase "Republican in name only"—referring to members of the Republican party who, in the writer's opinion, lack sufficient devotion to what the writer regards as the core principles of that party—go back many years. (Precisely what those core principles are seem to vary somewhat from writer to writer and from year to year.)
The earliest instance that an Elephind search finds of the phrase used in that particular way is from "A Strange Blunder," in the [Washington, D.C.] National Republican (January 26, 1875):
Next on the list, beginning from the same end, we find Mr. William Walter Phelps, of New Jersey, and Mr. Charles Foster, of Ohio, both of whom are Republican in name only, and both of whom have proved their treachery to party principles, to party friends and to the policy which can alone secure the success of the party. Clearly they are also inimical to the President [Ulysses S. Grant], to his Southern policy and to the success of the Republican party.
The Elephind search turns up several dozen instances of "Republican in name only" scattered over the subsequent decades, but the earliest instance of the acronym RINO that it finds is from Bronwyn Chester, "Republicans May 'Jump Ship,' not Vote in November," in the Daily Kent [Ohio] Stater (April 16, 2004):
Unlike Democrats, who often seem more like a gaggle of squabbling special interest groups than a political party, we Republicans don’t take kindly to those who break ranks. Over the years, this rather iron-fisted party discipline has borne valuable fruit, including the takeover of both chambers of Congress, the White House and a slew of state capitals.
But it has also resulted in a Republican propensity to push to the right of the next guy and to toe the party line, lest you be branded "a squish" or a RINO—Republican In Name Only. Dissension from basic party norms—let alone party leaders—is tantamount to heresy, so dissenters tend to keep mum.
A Google Books search pushes the earliest occurrence back a few years, to 1998, with one additional match each from 2001 and 2002. From The Economist (1998) [combined snippets]:
Mr [Richard] Riordan, the nominally Republican head of an overwhelmingly Democratic city, is so hated by his own party that he is denounced as a Rino (Republican in name only). Many of his friends would not bring their money or talents to the aid of the city if it meant running for office. But there is one important truth in the left's criticisms: the system is hopelessly unaccountable. The mayor is the first person to recognise this.
From "Don't Let the Failure to Defeat Marge Roukema Inhibit Future Attempts to Purge Liberals," in The American Spectator volume 34 (2001) [combined snippets]:
But the effort did affect Roukema. On the House floor, she approached colleagues who had received backing from the Club for Growth to inquire about what it would take to get straight with them. She modified her voting pattern, which now includes support for total repeal of the estate tax and—after years of opposition—for the requirement of a congressional super-majority to increase taxes.
The club now regrets it did not go after another RINO—Republican In Name Only—in the 2000 primaries: New York's notorious Rep. Sherwood Boehlert, whose voting record is far more liberal than Roukema's. His primary opponent, David Vickers, a conservative high school teacher, pleaded for club money but he was turned down because he did no seem to stand a chance.
And a year later, from Notra Turlock, Code Name Kindred Spirit: Inside the Chinese Nuclear Espionage Scandal (2002) [combined snippets]:
Warren Rudman, a former U.S. senator, was Clinton's appointee as chairman of the PFIAB [President's Foreign Intelligence Advisory Board]. ... In conservative political circles, he was commonly known as a RINO, "Republican in name only." He left the Senate years earlier, but continued to hang around Washington serving on prestigious boards and blue-ribbon panels. He was the perfect selection to head Clinton's PFIAB. As a nominal Republican he gave Clinton top cover, but was certain to defend the White House at the end of the day.
Clinton appointed Rudman to the post in 1997, so if Turlock's account is to be trusted, RINO was already a commonplace "in conservative political circles" by 1997. Turlock doesn't mention that Rudman was vice chairman and ranking Republican on the Senate's special committee investigating the Iran/Contra affairs in 1987 (nor that he behaved with exemplary honesty and fairness in the course of that inquiry). But it seems likely that the hostile conservative characterization of Rudman as a "Republican in name only" dates to that year.
William Safire, Safire's Political Dictionary, updated edition (2008) mentions RINOs in three separate places; but the dictionary doesn't provide a separate entry for the term, and both of the news articles it cites as using the term are from 2007.
In my memory, RINO emerged as a slang insult/acronym during Ronald Reagan's administration (1981–1989), but I haven't been able to find any mention of it from earlier than 1998, and I begin to suspect my memory of being unreliable on this point. So I have two questions:
How old is the first confirmed use of RINO as a pejorative slang acronym for a member of the Republican party who is (in the view of the user of the term) insufficiently conservative?
What are the circumstances surrounding that first recorded instance?