Romney would be horrible candidate for a number of reasons.
1) according to Gallup, 22% will not vote for a Mormon. This includes 18% of Republicans and 19% of independents. While Obama will not make a big deal directly, you can be assured that the MSM will be doing all sorts of exposes on Mormonism just to tell us what it's about.
2) ObamaCare- Kind of hard to run against ObamaCare when you invented it. Giving up your best issue is never a good idea.
3) "He looks like the guy who lays you off" - The main reason this recession has been so bad is that while other countries struggled to keep people employed, this one laid people off like mad. Obama made his fortune as a "turnaround artist" by going into companies, slashing employees, slashing benefits, and closing down plants in small towns that they depended on for jobs.
4) Phonier than silicon breast implants- Which Mitt Romney are we voting for again? The one who says that he's for protecting marriage from gay folks, or the one who said he'd be better on gay rights than Ted Kennedy. The one that is pro-life, or the one that is pro-choice? The one that denounced Reagan, or the one that raps himself in Reagan's mantle.
5) He's just not a very good candidate- What everyone forgets is that all the liberals who praise ROmney now will find a reason not to vote for him in November of next year. Just like they did with John McCain. Romney denounces Perry for being a "professional politician", but the fact is, the only reason Romney isn't is because he keeps losing elections.
He lost in 1994 to Ted Kennedy by 17 points. He barely won in 2002 against a non-entity, not even topping 50% of the vote. He opted not to run for re-election when he trailed Devall Patrick by double digits in 2006. In 2008, he lost most of the primaries and caucuses he ran in despite spending millions of his own money.