In the Washington Post yesterday, Paul Starr argued that it's really difficult to determine whether Hillary Clinton or Barack Obama would fare better in a general election. Ezra Klein made the same point, as did Kevin Drum. I find this maddening.
Look, it's obviously true that any guess about general election viability is speculative. You're predicting a future event, and you could be wrong. I think the New England Patriots would have a better chance of beating the New York Giants than would the Detroit Lions, but I can't be sure.
That said, the available data here is not very ambiguous. Hillary Clinton is a highly unpopular figure. In the last Gallup survey, 50% of respondents have a favorable view of her, and 46% negative. Sometimes her averages goes higher, but sometimes it veers into negative territory. Obama has very high ratings. In the most recent poll, 59% view him favorably, 32% negatively. The difference between plus 4 and plus 27 is enormous--a Detroit Lions v. New England Patriots-size gap.
On top of that, independents who vote in the primaries and caucuses have shown a very strong preference for Obama over Clinton. That is the closest available approximation of a swing voter. (Some Clinton supporters have pointed to her strength among lower-income Democrats in the primary, but a low-income Democratic primary voter is not the same thing as a working class swing voter.)
In 2000, Clinton ran five points behind Al Gore in the state of New York, and it's not like Gore was the most popular politician who ever lived. That's who she is--a figure who is disliked by pretty much everybody who isn't a sure-fire Democrat, and even some people who are. You can imagine Obama running a horrible general election campaign and becoming less popular. No doubt his favorable ratings would drop a bit in the face of Republican attacks, as would hers. But for him to become as unpopular as Clinton already is--without months of nation-side attack ads--is a worst-case scenario.
It's entirely possible Clinton could win if given a favorable environment and/or a sufficiently weak opponent. (Whether she could bring along as many Democratic Senators and Representatives is more doubtful, which is why so many red state Democrats are endorsing Obama.) And I'm not saying electability has to be a first-order consideration--if you think Clinton would be a much better president than Obama and are willing to accept a higher risk of a Republican winning, then go for it.
That so many Democrats think this question is complicated suggests to me that maybe people aren't good at assessing the popularity of their co-partisans. To Democrats, it's perfectly obvious that the strongest Republican nominee is John McCain. He polls very highly, everybody knows Democrats and Independents who like him, and so on. But Republicans are constantly debating this. You see Republicans spinning horror scenarios of a McCain nomination leading to a splintering base or depressed turnout. To Democrats it's bewildering that they even debate this. Lots of Republicans feel the same way about the Clinton/Obama electability debate.