Out of all the Democrats

[The Age Wave]

This is running on a lot of assumptions. I'm not entirely sure Romney could even clinch the nomination, let alone actually defeat Barack Obama in 2012. If Romney has an okay term at best, horrible term at worst, he would lost to just about any sane Democrat. Bernie Sanders and Al Franken would lose, but Hillary Clinton, Joe Biden, Bob Casey, etc. could all crush Romney head to head unless Romney's term is exceptional.
If Romney's term is great, which I'd doubt, Mark Warner and Evan Bayh would still make the race extremely close and either would still be favored to win, from my analysis. Romney could actually build numbers in the Northeast, but he'd lose enough votes elsewhere that it would destroy his chances. Getting 45% in Maine is pointless if you're a Republican getting 45% in Virginia or Ohio.