The other factor worth weighing for the general election in 2008 seems to me to be the Congress. If voters assume it will be Democratic, and that seems a pretty solid assumption right now, that changes a lot of things. It obviously favors a Republican who can work with Democrats. Romney will tout his Massachusetts experience, but he has so alienated those he once appealed to that he would be instantly rebuffed. Rudy's diplomatic skills need no introduction; he has none. Huckabee could point to his work with Democrats in Arkansas but that would sound a lot like Bush in 2000 and look where that got us. So, again, McCain is the obvious choice for a general election. Independents could back him on immigration, fiscal tightening, and climate change. The base would back him in order to save face in Iraq.
On the Democratic side, again, it seems to me a Democratic Congress makes Obama more viable. Centrist voters will worry about electing Clinton and a Democratic Congress. Everyone will remember 1992 - 1994. Obama is both a good negotiator for a Republican Congress, however unlikely that now seems. But he also seems independent enough to handle a Democratic Congress a little at arm's length. Constant atacks from Krugman and the netroots might even help him pivot nationally. All this assumes, of course, that there isn't a Democratic progressive tide that actively wants a new war on conservatism. I think such a tide exists, but doesn't get over 40 percent. Obama is the bridge to a majority for change. Clinton is a bridge too many people have already burned.