The Hotline's Chuck Todd says there is "plenty of evidence" that President Bush was the deciding factor that cost Republicans control of the Senate:

There's plenty of evidence to suggest that President Bush may have been the deciding factor that killed the GOP's momentum in some key Senate races over the last week. One Republican consultant is convinced that Bush's last-minute visit to Missouri on behalf of ousted GOP Sen. Jim Talent did the incumbent in. According to the network exit polls, Democrat Claire McCaskill crushed Talent among those late-breaking voters who decided in the final three days (a full 11 percent of the electorate). Bush also made a last-minute trip to Montana, where anecdotal evidence indicates the president's rally for Republican Conrad Burns stopped the incumbent's momentum in Billings.

Todd cites the exit polls for Missouri, which do indeed show late breakers going to McCaskill, though it's impossible to say that had anything to do with Bush's visit. The data is, at best, inconclusive: Bush had a 45% job approval rating in Missouri, and close to half of those who voted (46%) said Bush did not play a role in how they cast they're vote.

With regard to Montana, Todd cites "anecdotal evidence" to support his argument that Bush somehow stopped Burns' momentum, when in fact those very same exit polls in Montana show just the opposite:

It looks like the "rule" that undecideds always break disproportionately for the challenger proved true Missouri, but not in Montana. In any case, it's impossible to say how much Bush's last-minute visits had to do with the outcome in either of these races.