In the Republican primaries, Mike Huckabee upsets John McCain in South Carolina. Romney wins Florida and after a prolonged battle with Huckabee and a weakened McCain emerges as the Republican nominee. He picks Gov. Tim Pawlenty of Minnesota as his running mate.
The Democratic Primaries go as they do in RL, and Obama is nominated and selects Joe Biden as his running mate.
What happens?
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My prediction:
The eventual result is actually quite similar to real life. Romney does better in Michigan and Minnesota, as well as slightly better in the Northeast and in the Mormon West. But turnout throughout the South is low due to low enthusiasm among evangelical voters. Arizona is also extremely tight, as Obama contests it with McCain out of the race.
Still, in the end, the popular vote winds up being quite similar to RL, with only Missouri flipping to Obama. (I see Romney ultimately outperforming McCain in the popular vote slightly, due to a better response to the financial crisis.)
Obama/Biden: 376 E.V., 52% of the vote
Romney/Pawlenty: 162 E.V., 46% of the vote