First Read makes an interesting point:
How much power does McCain have in this? If he comes out against the bailout, he probably can kill it or radically alter it. But then the Republicans own the alternative, right? What a mess.
I'd put it slightly differently: McCain can probably kill or alter the proposal. But, if he does, he delays an end to the financial crisis. And the longer the crisis continues, the worse it is politically for him.
So McCain's damned either way. He either gets on board the Bush/Paulson plan, in which case he ties himself to a spectacularly unpopular administration. Or he derails the plan and makes a play for voters' economic hearts and minds, but ensures that the financial crisis remains topic A, making the political terrain about as unfavorable as it could be.
That's probably why you see McCain tying himself in all sorts of knots here.