My thoughts are that the answer to this question is heavily dependent on the way the question's asked. For example, here's a poll from Pew:
http://people-press.org/reports/display.php3?ReportID=375in which the following question was posed to likely GOP Iowa caucus-goers:
Do you think the US should keep military troops in Iraq until the situation has stabilized, or do you think the US should bring its troops home as soon as possible?
until the situation has stabilized: 81%
bring troops home immediately: 15%
What gives? Why would there be such a huge difference between "as soon as possible" and "within the next six months"? It's because you have to be careful about what question you're asking. The relevant issue isn't over the timeframe for withdrawal, it's what conditions you want to withdraw in. The people who say they want to stay until "the situation has stabilized" presumably believe there's some goal that could still be achieved in Iraq, and want to stay until that goal is achieved. If you asked them follow up questions, you'd probably find that they're hoping this can happen within the next year or the next six months (and that's why many of them say they "want to withdraw" within six months), but if the conditions are not met, then they *wouldn't* favor withdrawal.
On the other side, there's probably a significant majority of the country who don't think there's anything left that's achievable in Iraq, and just want to leave right now, regardless of the conditions. But while this is likely a majority of all voters, it's probably still a small minority of Republicans.
