Of the current GOP field, only Romney and Huntsman will have difficulty beating Obaman. I don't think Huntsman will be the nominee. Romney could well be the Bob Dole of the early 21st century. And the results will be the same.
I'm not sure if Romney can win the nomination. If he does, I'm pretty sure Obama will get 4 more years.
My first choice is Palin. My second choice is probably Herman Cain. Beyond that, I think only Romney and Huntsman will lose my vote in the General election. If either of them are on the Republican line, I'll look at third parties, leave it blank, or write in Palin.
I think Romney’s strongest support will be Dems in open primary states. I’m not knowledgeable enough to know if that will put him over the top or not.
In a 3-way race I see:
Obama>Romney>Tea Party
With Baraq pulling off a Clintonian plurality win.