Of all the mentionees here on the Republican side, I think Daniels would be the best candidate and a serious threat to Obama. He is a true believer right wing conservative but he also understands that government is needed — and he doesn’t go out of his way to offend people the way Chris Christie does. And Indiana is in pretty good fiscal shape today; he would certainly carry his home state, which Obama carried in 2008. (The last time that a candidate lost his home state but still managed a victory was 1916, and there is no way that Mitt Romney is going to carry Massachusetts.) The trouble for Daniels is that the reason Indiana is in pretty good fiscal shape today is that Daniels raised the sales tax, and that will kill him with Republican primary voters. The Tea Partiers who want to destroy government rather than to make it work better won’t accept that.
On the other extreme, I suspect that Trump or Palin might lose 49 states to Obama. (Trump could lose 50.)
Looking at things state by state, it is pretty hard to see Republicans switching enough states from 2008 to defeat Obama in 2012 — and Republican governors in Wisconsin, Ohio, and Florida are a major reason for that. Paul Ryan’s attempt to end Medicare as we know it has probably killed Republican chances in Florida.