I'm tired of people here (and throughout the political spectrum, for that matter) pushing the meme that Republican X "can't defeat Obama". So I'm going to go through several candidates and give reasons why each one can defeat Obama:
Mitt Romney can defeat Barack Obama.
Yes, Romney gives up the natural Republican edge of hanging ObamaCare around his neck thanks to Massachusetts and RomneyCare. However, that does nothing to alleviate Obama's other weaknesses. Obama has run the national debt up to an insane degree, and Romney can hammer him on that repeatedly, playing up his own managerial skills to good effect. In addition, Romney has a strong base dating back to 2008 that will do him well. Having already run a national campaign before in 2008 will also do Romney well; it's no coincidence that most Republicans that won a Presidential election had already run for President previously (Bush Sr., Reagan twice, Nixon).
Tim Pawlenty can defeat Barack Obama.
Pawlenty has taken some problematic positions in the past, like cap and trade, however he's done his best to distance himself from those; if Obama tries to use those against him, he can take the reasonable position that he's learned from his mistakes, unlike Obama. Pawlenty's biggest challenge at this stage is that he's mostly unknown nationally, so he needs to define himself to the general public before the media and the Democrats do it for him. Having been governor in a blue-purple state means that he should already have at least one state in the bag for him that didn't go for Republicans in the last three presidential elections, which could be the difference in a close race.
Jon Huntsman can defeat Barack Obama.
Yes, he was Obama's ambassador to China, but he can answer any negative criticisms by pointing out that ambassadorships aren't political positions, and how he started out as a staff assistant in Reagan's White House. Huntsman can point out his record of cutting taxes in Utah, and how Obama really hasn't kept his tax pledge. As with Pawlenty, he'll need to define himself before the Democrats do.
Sarah Palin can defeat Barack Obama.
Yes, Palin doesn't do very well amongst independents right now. But then again, neither does Obama. Independents that think poorly of both Palin and Obama are likely to not vote at all or vote for some third party, which reduces the election to an exercise in turning out the base. In that situation, you have Palin, who energizes the base better than anyone, against Obama, who has spent the last two and a half years disappointing his base. Republicans have got to like the odds in that situation.
Herman Cain can defeat Barack Obama.
Sure, Cain has never ran a national campaign, and he's never held any significant political office before-- but in an election like this, being an outsider has to be considered an advantage, and it's not like he doesn't have any executive experience, having been a CEO that turned a company around. Yes, he will make mistakes, novice mistakes even. But don't discount the 'black-on-black' nature of an Obama vs. Cain race; there will be a significant number of independents that voted for Obama previously that are disappointed with him, and the idea of saying 'I voted for the first and the second black President' will appeal to them. Cain also has better rhetorical skills of anyone potentially running (save Palin), and that will work well to fire up the Republican base even more than they already are.
Ron Paul can defeat Barack Obama.
Okay, not all the current field of Republican candidates can defeat Obama. If by some fluke Ron Paul were to get the Republican nomination, the media would very quickly destroy him by pulling out issues of the Ron Paul Newsletter that have racist content. The first black president running for re-election against a media-confirmed racist? It would make 1964 look like 2000. The only question would be, would Obama carry all 50 states or not.
So with the exception of Ron Paul, I think any candidate that's good enough to win the Republican nomination is good enough to defeat Barack Obama. What we should be concerned about is choosing the best candidate, not trying to play strategic games that will only work to our disadvantage. Heck, if it hadn't been for the financial meltdown, we likely would have dragged John McCain's sorry carcass across the finish line and into the White House. Let's worry about getting the best Republican possible to be our standard-bearer in 2012 and retire the 'unelectable' myth once and for all.