Jon Chait notes that John McCain has engaged in some pretty astounding policy meandering over the years in a way that makes it absurdly hard to tell what he would actually do as president:
Determining how McCain would act as president has thus become a highly sophisticated exercise in figuring out whom he's misleading and why. Nearly everyone can find something to like in McCain. Liberals can admire his progressive instincts and hope that he is dishonestly pandering to the right in order to get through the primary. Conservatives can believe he will follow whatever course his conservative advisers set out for him and will feel bound by whatever promises he has made to them. Even the ideological tendency McCain is most strongly identified with--neoconservative foreign policy--is, as John B. Judis explained in The New Republic, a relatively recent development: McCain originally opposed intervention in Bosnia and worried about a bloody ground campaign before the first Gulf war (see "Neo-McCain," October 16, 2006). McCain's advisers include not only neoconservatives but also the likes of Henry Kissinger and Brent Scowcroft. It would hardly be unimaginable for McCain to revert to his old realism, especially if Iraq continues to fail at political reconciliation. He could easily be the president who ends the war.
The amazing thing about McCain is that his reputation for principled consistency has remained completely intact. It is his strongest cudgel against opponents. Wall Street Journal editorial page columnist Kimberley Strassel recently gushed that McCain is "no flip-flopper." "Like or dislike Mr. McCain's views," she added, "Americans know what they are." Then, in the very next paragraph, she wrote that McCain will now be "as pure as the New Hampshire snow on the two core issues of taxes and judges" and that "[t]he key difference between Mr. McCain in 2000 and 2008 is that he...appears intent on making amends" to conservatives.
It seems to me that one's best bet under conditions of uncertainty is to assume that politicians will, in fact, implement the agendas they're campaigning on. So if John McCain says he'll resist the repeal of the Bush tax cuts he probably will, as president, resist the repeal of the Bush tax cuts. But what's really disturbing about McCain's many flip-flops is their often obscure motivation. His far-right swing on national security has wound up serving him well in the 2008 GOP primary, but it actually took place back in the mid-to-late 1990s, so it's not the result of straightforward political calculation. Similarly, his big veer to the left in 2002-3 didn't seem to have much of a root cause beyond personal pique -- he was pissed at George W. Bush and at the time Democrats were pushing a pretty modest, tepid agenda so McCain embraced large chunks of it to spite his rival.
And as Chait emphasize, McCain actually denies that any of this swaying to-and-fro ever took place, so he doesn't have any kind of story to explain what it's all about.
If you find the vague themes of nationalistic collectivism running through McCain's career to be appealing, maybe this meandering on substantive issues looks reassuring to you. If you don't see the appeal, then I think it looks frightening. But from a pure electoral perspective, the really dangerous thing is that it's hard to imagine McCain (or anyone) ever doing anything more brazenly flip-floppy and dishonest than his shifting story on taxes and yet it hasn't changed his reputation at all. All we can take from that, I think, is that McCain can say literally anything he wants and the press will still say his shit smells sweet. There's the old joke about Bush saying the earth is flat and the papers reporting at as "Flatness of Earth Disputed" but if McCain were to say it you'd just get a "New Study: Earth is Flat" headline.
Still, in terms of what McCain would actually do as president, the fact of the matter is that we just have very, very, very little evidence. Under the circumstances, the best thing to do is probably to assume he'll do what he says he's going to do -- cut taxes, curtail spending, and bomb Iran -- but there's sort of no telling.