Andrew Romano sees a flaw:
Let's assume that voters buy McCain's basic premise that it'd be good to have a divided government and are willing to vote in part to preserve one. It's still not clear why they'd vote against the presidential candidate that they prefer--56 percent are either “optimistic or confident” or “satisfied and hopeful” that Obama would do a good job, while only 44 percent say the same of McCain--instead of simply voting against their local Democratic congressional candidate. McCain has an admirable history of crossing party lines, and he's probably the Republican best equipped to get things done in what promises to be a massively Democratic D.C. Still, he's hoping voters will base their presidential preference on the likely composition of Congress. That seems a little backwards.