At a rally a few weeks ago in Newport News, Obama criticized McCain's economic program, claiming that the average CEO would get a $700,000 tax break, and then added: "Not only is it not right, it ain't right." Apart from the obvious "just folks" implications of the register shift, the line exploits a subtle distinction in evidentiality that Tom Wasow pointed out to me some years ago, which I worked into a "Fresh Air" piece back in 2002:
A while ago my Stanford colleague Tom Wasow sent me an article from the Chronicle of Higher Education that quoted a dean at a prestigious Eastern university: "Any junior scholar who pays attention to teaching at the expense of research ain't going to get tenure." That ain't was a nice touch: it made it clear that the dean's conclusion wasn't based on expert knowledge or some recent committee report — it was something that should be clear to anyone with an ounce of sense.
That's the message that ain't conveys in all those common expressions like "It ain't over till the fat lady sings" or "If it ain't broke don't fix it" –ain't tells you that you're dealing with a nitty-gritty verity that you don't need a college education to understand.
The interesting thing about this use of ain't is that it pops up in contexts where an educated speaker would be unlikely to use a run-of-the-mill nonstandard form like "He done it" or "They didn't get none." In fact you hear this evidential ain't a lot from political figures in speeches and interviews, and not just in idioms like "it ain't over till it's over" and "say it ain't so":
And so, kind of Psychology 101 ain't working. It's just not working. I understand the issues, I clearly see the problems, and I'm going to use the NIE to continue to rally the international community for the sake of peace. George W. Bush
I think, again, that we've got to do the best job we can of trying to facilitate some kind of an agreement between the Israelis and the Palestinians and getting back to Tenet and Mitchell. It ain't easy. It's one of the toughest, most difficult, impractical problems I've ever seen. Dick Cheney
Well, when a politician gets corporate contributions and then writes an earmark for that corporation, which is not competed or any other virtue that I know of, that's not illegal, either. OK? But it ain't right. John McCain
This ain’t your father’s Republican Party, by the way. This is a different Republican Party. Joe Biden
It ain't gonna happen. Tony Snow
My guess is that this usage accounts for the great majority of the occurrences of ain't that you see in print and other relatively formal contexts, outside of direct quotations of nonstandard speech. That may be one reason why the word is likely to remain in the penumbra of standard English, despite its phonological naturalness as a contraction. If ain't ever became respectable (as it nearly did in the nineteenth century), educated speakers would no longer be able to use it to deliver themselves of a homey-sounding truth.