1. Sarah Palin (believe it or not): Maybe this will change during the campaign season but ever since 2008 and the last campaign season it seems like Obama lacks the willpower to fight and that as James Carville said, he's no longer "balsy" (a Southern term basically saying he has no balls and doesn't stick to his guns). Palin could probably say whatever she wanted about him and he may not "refudiate" it, or if he does, he wouldn't be as aggressive in his fighting tactics.
2. Tim Pawlenty: He doesn't compromise much with Democrats much but appeals to Republicans and independents because of his right of center policies which appeal to this country which is still very conservative, and as Obama showed in 2008 all you need is independents and your base.
3. Scott Brown: Same as Tim Pawlenty but he actually does compromise with Democrats, once in a while but not often and still is riding off the steam of his 2009 special election win to replace Ted Kennedy in the Senate.
4. Marco Rubio: He appeals to the Spanish-speaking community who is still waiting for the Democrats to pass the DREAM Act (gives illegal immigrant teens/kids who were brought to the US illegally when they were younger without consent by parents citizenship) and has the speaking abilities of Obama with an ability of eloquence.
5. Gov. Christie: Despite continued denials, just see Scott Brown's response.
And that's not it.... People like NY Mayor Michael Bloomberg (who has compromised with both sides and stands with both Dems and Repubs on different issues), Joe Scarborough (who has gained political clout from his non-partisan rhetoric with Republican leanings on "Morning Joe" on MSNBC and is also rumored to be running for VP with Mike), Howard Dean (who believes the Prez isn't being Democratic enough) and Donald Trump (who is a pure conservative that is also popular from his NBC show, "The Apprentice") could all be in contention to run for office either as independents or during the Republican primaries.
Now Scarborough and Bloomberg have declined they are running, but Trump hasn't put it out of reach. Dean most likely wouldn't run because he wouldn't want to cause any division in the Democratic Party.
Considering at least 2 of the independents run, it could be a 4-way race in 2012. Those two would most likely be Bloomberg and Trump who have nothing to lose unlike Scarborough who has a television platform he probably doesn't want to lose. It could also be a 3-way race between potentially a Palin-Rubio card vs. Bloomberg (who has the highest prospect for running out of all independents because of his age, experience, popularity and appeals to Obama supporters) and most likely a female running mate (to steal the females from Palin) and an Obama-Biden card on the blue side.
But what if Palin loses the Republican nomination? Could she still run with Tea Party support as an independent? It is still a possibility because remember Republican primaries (how to decide Republican nominee) are only voted on by registered Republicans. Bloomberg could also possibly run which would make it Obama-Palin-Some Repub-Bloomberg for the presidency in 2012. Or it could still be a 3-way race between Obama-Palin-Some Repub.
Or if no independent runs and Palin wins, it could be straight up PALIN VS. OBAMA.
All this speculation and conjecture is crazy, right? Well let me tell you how easy it really is for Obama to win. "It's the economy stupid!" as James Carville said during the Clinton administration. (Why doesn't Obama hire him as his chief of staff?). Americans that aren't involved in politics mostly care about what have you done for me lately. If the economy is back, believe me you can say President Obama all the way until 2014. I guarantee it.