Anyone but McCain equals Dem landslide. A different Democratic candidate shifts a few close states here and there, but doesn't change the map fundamentally. (The real struggle for the Dem nomination was for who becomes the anti-Hillary early on... and that field was arguably wide open, so there's a lot of quasi-plausible contenders here.)
That's the CW but I'm skeptical. As I mentioned elsewhere, Romney and Huckabee had their own sets of strengths and weaknesses and nobody can say for sure how the campaign would have played out if one of them was the nominee.
Romney wouldn't have been so massively outspent, had a far better grasp of economics than McCain and his campaign would have been arguably much more efficient.
OTOH, his corporate past, his phoniness and his Mormonism would have hurt him.
Huckabee was the most charismatic of the three, his economic populism was well suited to the 2008 campaign, he could have pummeled Obama for his bailout vote and his candidacy would have kept the republican base enthusiastic.
OTOH, his lack of national security credentials, his overt religiosity and his inability to raise funds were serious cons.
