Republicans, reports Jennifer Rubin, are nervous that their weak candidate field is going to blow a great chance to knock of President Obama:

If you talk to Republicans in D.C. -- or in Israel, for that matter -- the overwhelming sentiment about the 2012 contenders is a mix of un-enthusiasm and nervousness. Republicans genuinely believe that the economy and Obama's failure to exert leadership on our fiscal crisis provide an opening for Republicans to do the unusual -- defeat an incumbent president....
There is widespread conservative fretting within the Beltway. Could a golden opportunity slip through the GOP's fingers?

I don't really think it's a golden opportunity. I think it's more of a long-shot opportunity. Incumbent presidents usually win. They can lose if they're quite unpopular, but Obama isn't:

PPP
In 2008 Barack Obama won nine states and one electoral vote giving Congressional district that had gone to George W. Bush in 2004. We've now polled every single one of those over the last three months except for Indiana, where we can't do one because of restrictions on automated polling in the state. Across 36 horse race match ups against Newt Gingrich, Mike Huckabee, Sarah Palin, and Mitt Romney in Florida, Ohio, North Carolina, Colorado, New Mexico, Virginia, Iowa, Nevada, and Nebraska's 2nd Congressional District Obama is 36 for 36. If he stood for reelection today against one of the current Republican front runners Obama would almost certainly win the same number of electoral votes he did in 2008, if not more. ...
Republicans need two things to happen over the next 20 months if they're going to beat Obama and time is one thing they have on their side- they need a much stronger candidate to emerge, whether it's someone outside this top 4 or someone inside this top 4 successfully remaking their image, and they need Obama's numbers to get back in negative territory and probably by a good amount- somewhere south of 45%. 

Lots can happen between now and November '12. But the key factor is the economy, and it's a lot easier to imagine that getting better than it getting worse.