I would say this is the map of Obama vs. Generic Republican. Certain candidates' strengths or weaknesses would help them to exceed this map's expectations or otherwise fail to reach this 303-235 loss with dignity.
So far, "Generic Republican" seems to be running stronger than any well-known Republican. I can see regional weaknesses for every one of them (Romney would do better in the North but at the expense of a Southern state or two that the GOP can't afford to lose; Huckabee would do badly in the North, perhaps picking up North Carolina (which wouldn't be enough) but losing the Dakotas). That's before I talk of candidates on the fringe of either ability (Palin), skills running a national campaign (Gingrich --- think of Gerald Ford without the geniality), or near-unknowns. In view of how Pawlenty (lapsed Catholics would lose places with lots of Catholics) and Santorum do in their "own" states, you can write them off now. At that, I don't see how a fairly-unknown Senator from a very Red State (Thune, South Dakota) that is no microcosm of any large region of America has a chance.
"Generic Republican" and "Generic Democrat" go into hibernation at the time of the first primaries and stay in hibernation until the electoral season is over. In contrast, we all know President Obama and his political weaknesses (including geographic vulnerabilities) very well. I think that Republicans are looking for personal electoral strengths of some unidentified Republican whose political acumen will overwhelm President Obama or that President Obama will be an abject failure.
I have a problem with anyone winning 335 or so electoral votes for the simple reason that nobody wins with between 57% and 65% of the electoral vote. The one in the lead plays it safe with the political equivalent of the Nickel Defense, consolidating states that might be shaky while abandoning those on the margin of victory; the one behind tends to take chances that either make the election closer or turn the election into a blowout. Obama was on the low end of the blowout zone in 2008 and will likely either
(1) lose by a small margin (235-268 electoral votes)
(2) win with fewer than 315 electoral votes, or
(3) win 355 or more electoral votes (the sky is the theoretical limit, especially against some incompetent campaigner)
315-355 would be unprecedented since 1900.
As an illustration, if he won the states that he won in 2008 and no others he would win 359 electoral votes in 2012, or 358 if he lost NE-02 or the state went to winner-take-all.