Thomas B. Edsall previews a general election between Clinton and McCain or Obama and McCain:
If the general election is between Obama and McCain, both candidates will be under pressure to lessen partisan tensions in order to keep the support they have among independents and voters of the opposite party. The battle would likely be for the center, or swing, electorate, making the polarizing strategies that characterized the 2004 election more dangerous, because such maneuvers could alienate moderates. Conversely, McCain and Clinton would both see more advantage in pursuing intensely partisan tactics designed to build turnout and enthusiasm among core loyalists. Polling data show that Republicans in a Clinton-McCain contest are much less willing to consider voting Democratic than they are if the race pits Obama against McCain.
One possible advantage McCain may have in a general election match-up versus Obama is the Latino vote, which, while not making up an especially large portion of the electorate, may embrace McCain over the senator who arguably blew up last summer's immigration deal. (Of course, that fact may also help Obama among some anticomprehensivist Republicans. You never know!)