I'm not sure I really buy John Judis' argument here. First, he observes:
Hillary Clinton won the big states she had to win, and arrested Barack Obama’s momentum, but she is going to have problems with white male voters. Obama is having trouble with white working-class voters and Latinos.
Then he goes on to extend this analysis to the general election, arguing that both Clinton and Obama have critical weaknesses. The trouble here is that Judis' method is going to reach the conclusion "Party X is Doomed" any time Party X has two fairly equally matched contenders. Even if the two contenders are both very strong, each is going to look "weak" among whichever groups of voters prefer the other candidate. Conversely, if there are two very weak contenders then they're both going to look "strong" within the demographic groups where their rival is especially weak.
To me, most indications are that the Democrats have two strong contenders. Consider that in Missouri about 552,000 people came out to vote in the GOP primary -- a primary that all three candidates seriously contested. By contrast 800,000 people came to vote on the Democratic side. If you put all five candidates into a single election, Hillary Clinton's second place showing of 395,000 would have trounced John McCain's 194,304 for third place. Both candidates, in short, are good at appealing to large numbers of voters and getting them to show up.