PPP now has him trailing Michelle Bachmann in a national head-to-head matchup. Check out this part about Romney's support for the individual mandate:

Only 17% of Republican voters say they'd be willing to vote for someone who had supported an individual health insurance mandate at the state level, compared to 66% who say they would not be willing to support such a candidate. The funny thing about that is Romney's getting 17% right now with that latter group of voters and his favorability with them is 49/33. Your average primary voter isn't tuned in enough to the race right now to know the specifics of Romney's record.

I don't think the wording of this finding should be taken literally. When 66% of Republican primary voters say they wouldn't be willing to support a candidate who had supported the individual mandate, I wouldn't take it to mean they'd never vote for such a candidate under any circumstances. It does, however, signal a serious vulnerability. And, as PPP says, Romney's opponents haven't begun to exploit that vulnerability yet.

It's also worth noting that, as Benjy Sarlin reports, 70% of Romney's funds have come from maxed-out donors, while only 6% of Bachmann's donors have maxed out. She's building a large-scale small donor base like Barack Obama had in 2007-2008, and Romney is not.

I am waiting to see what happens with Rick Perry's apparently-likely entry into the race. To be honest, I have zero sense of how he'd perform.