McCain doesn't like the bailout plan.
He has come to Washington to scuttle the consensus and start over.

Currently everyone resents his presence, but he is going to make himself a permanent fixture. Congresspeople will continue to be irritated for a while, but McCain will eventually get the support of the Republicans who don't like the bailout idea.

McCain will work with Republicans to develop a counterproposal. Democrats will have to wait for it because they can't approve a bailout without bipartisan cover.

Eventually bipartisan negotiations will start up again. The plan that comes out of these negotiations will look different than the consensus that was just scuttled. It may not bear much resemblance at all to the Treasury proposal.

Democrats will say that John McCain stepped in to take credit for something that was already happening. John McCain will point out that he was able to win concessions, and that the new plan looks different from the old plan.

Depending on how big those concessions were, and how different the new plan really is, and in which ways it is different, the public may decide that McCain's influence was helpful. If they do decide this, then McCain may get a boost, despite all the preening and the presumptuousness.

Substance will drive the political outcome. If McCain causes substantive changes that are really beneficial, he will go up in the polls. If he does not, he will be justly rebuked for causing needless trouble in a moment of crisis.