Surprisingly, McCain has broken out of the pack (with Romney nipping at his heels). I'd argue that McCain, of all the Republicans, has the best chance of winning the general election.
If you look at McCain's positions on the major issues - the Economy and Energy, Health Care, and Immigration - you can see that his positions are not so far off from the Democrats. He has voted against drilling in Arctic National Wildlife Refuge and wants to reinvest oil profits into nuclear and alternative energy sources, for instance. (A position very different from Romney's.) His immigration policy is less xenophobic and more pragmatic than even Clinton's.
McCain is most notably distinct from the Democrats in his position on the Iraq War. Yet if the surge continues to lower the level of violence, his initial advocacy for it will, at a minimum, become less of an issue to a nation that has already swung from broad support to broad opposition and may swing yet again.
I love the fact that pundits keep talking about shifts in momentum, like first-week physics students trying to describe the movement of a pinball. The difficulty of campaigning on Super Tuesday without broad support has made casualties out of Edwards and Giuliani. Next week, it is quite possible that McCain will have emerged as the front-runner. I can think of no other candidate that the Democrats should fear more.
Feeling oddly bold about 15 months ago, I predicted the presidency through 2020. Right now, my prediction of McCain for the next four years seems more probable than it has in months.