President Nixon declared, "I am not a crook."
President Nixon declared: "I am not a crook."
President Nixon declared "I am not a crook."
Do you agree that using the comma and the colon in the sentece above is "completly pointless" as one of the many manuals dedicated to the problem of quotation marks asserts? According to this manual the first two sentences are the examples of "bad style".
I don't have a definitive source to base this on (other than John Lawler's insightful comment). But, I have a strong intuition about the usages listed here.
For the sentence structure given, no comma or colon is needed. The quote is entirely "in-line" with the rest of the sentence.
If the sentences were modified slightly so that the logical, or oral, flow changed then I would treat it differently. For example I would use a comma in this sentence:
As President Nixon declared, "I am not a crook".
Picture yourself reading that sentence aloud to an audience; to make clear the grammatical structure of the sentence you use an "intonation contour". The comma would server the same purpose in writing, by separating the quote from the narrative commentary.
I would use a colon in this sentence:
President Nixon, who declared: "I am not a crook".
Again the structural change inserts narrative commentary into the sentence. This specific quote contained in the sentence has now become an example, an instance of declared from among many declarations. As such I would now treat the second clause of the sentence as a list (which just happens to be a list of one item).
