Fun with QBasic!
Let's start with Obama's state results in 2008 used on a map with the 2010 census numbers. This is what would happen if he carried all the states he carried in 2008 but didn't pick up any. (For simplicity's sake, I'm ignoring the single vote in Nebraska he got.)
Given the present conditions, it's likely that he will not do as well as he did last time. If his percentage of vote totals went down equally among all states, one state after another, starting with North Carolina, would turn red. Still, he could give up almost 9% and still have this map:
The next state to flip would be Colorado at 8.95%, which would put him under the limit.
But it's not likely that each state would swing the same amount. If we start with the premise that Obama can hold all the Kerry states (if he can't, he's in deeper shit than we thought) then the addition of Florida alone would put him over the top:
So would the combination of Ohio and Nevada (or Ohio and Iowa), if only by the slimmest of margins. Better hope for no rogue electors!
I think demographic trends in Virginia might be more favorable to him than Ohio, but if he takes Virginia while losing Ohio, he needs two more smallish states to make up the total. Any two of Nevada, Iowa, and New Mexico, for example, would do the trick.
Demographic trends seem to favor him in North Carolina too, but probably not by enough, as he just barely squeaked it out last time. If he can hold N.C., his job becomes a lot easier, just the Kerry states plus Va. then become enough.
And, of course, the obligatory tie scenario, with Obama holding Ohio and New Mexico.