The "electibility" argument among Democratic presidential candidates is complicated enough from the get-go, as illustrated by a recent exchange between Jonathan Chait and Ezra Klein about the general election strengths and weaknesses of Obama and Clinton, which pretty much covers the waterfront of informed speculation.
But two new factors are pushing the electibility debate in new directions. The first is the re-emergence of John McCain as the Republican front-runner. He's the only GOPer in shouting distance of the two Democratic front-runners in general election trial heats, and actually runs ahead of Clinton in some. And there's a reason for that: aside from his famous appeal to independents, and the media adulation he enjoys, he's the one candidate hardest to typecast (except on the subject of Iraq) as mired in the same ideological delusions that gave us the Bush administration. His one major electoral weakness, the hostility of movement conservatives, won't be much of a problem in a general election (and they seem to be coming around; elite conservative attitudes towards McCain at the moment strongly resemble those of establishment Democrats towards Howard Dean just prior to Iowa in 2004--resigned).
The second factor, though, cuts in the opposite direction. It's now virtually certain that the economy will be in recession during the critical period prior to the general election. This will represent a real anchor on the Republican ticket, regardless of its composition. And this is not a subject conducive to any McCain Magic. The Arizonan's economic message is basically one of fiscal austerity seasoned with a commitment to heavy defense spending. It's hard to imagine a prescription less well suited to hard times.
This problem may catch up with McCain in the primaries; Mitt Romney would be insane not to exploit the advantage of being both more knowledgable and more conservative than McCain on economic issues. But if McCain does win the nomination, all the talk about service and courage and straight talk won't matter much to people who are worried about their jobs, their pensions, and their homes.