I think Romney’s strongest support will be Dems in open primary states. I’m not knowledgeable enough to know if that will put him over the top or not.
In a 3-way race I see:
Obama>Romney>Tea Party
With Baraq pulling off a Clintonian plurality win.
He could also get significant support in states with closed primaries, if there are no serious primary challengers to Obama. States like Florida, where there are a lot of Democrats who could register as Republicans before the presidential primary, could see significant cross over. The timing on Presidential primaries and down ticket primaries in Florida allows one to switch parties, vote in the presidential primary, switch back, and still vote in the Democrat primaries for Congress, state and local offices.
If Hillary or some other serious player challenges Obama on the Dem side, I see a good opportunity for a conservative on the Republican side. A Palin v. Hillary general election could be very interesting.
If Obama has no challenger, Romney's chances for the nomination go up, and Obama's reelection odds get better.