I'm not sure I understand Marc Ambinder's logic here:
The data from last night suggests that voters believe that Hillary Clinton's argument about Barack Obama's general election viability will remain valid until Obama renders it invalid. He did poor relative to Clinton among most discernible swing groups despite a massive, $12 million, six-week investment. The argument, incidentally, isn't that because Obama didn't win Pennsylvania in the primary, he can't win it in the general. It's that the coalition Obama is building in these states cannot, without a significant modification, give him victory in the fall. The corresponding argument is that it will be easier for Clinton to expand her coalition.
It seems to me you need to cut this closer. Certainly one easy opportunity Clinton has to expand her coalition is that versus John McCain she would pick up all these African-American Obama supporters. But conversely, I don't see Hillary Clinton's feminist supporters suddenly deciding that they want to see John Paul Stevens replaced with an abortion-banner. The theory here seems to be that Clinton's strength among white working class Democratic party loyalists will translate into strength among white working class non-loyalists. But there's no evidence for this theory -- both Al Gore and John Kerry formed "beer track" primary coalitions and then went on to perform terrible among white working class voters overall. This is an electoral challenge for either candidate, but it really is an electoral challenge for either candidate. Young, anti-war Obama supporters will back Hillary over Old Man War and Clinton's supporters in the public sector unions (notice all the AFT and AFSCME signs behind her at every rally) will easily sign on with Obama. Ultimately, for the party not to unite behind the eventual winner there would need to be a much more serious substantive cleavage between them.
Ultimately, the only thing we really know about either candidate's base of support is that Obama's is slightly larger, and that he's demonstrated more ability to expand it over time through campaigning. Either way, though, I think Democrats can be fairly confident. What voters "know" about the three candidates right now is that John McCain is a war hero maverick, Hillary Clinton is a castrating harpy, and Barack Obama is a radical black Muslim and given that . . . the race is about tied. As Ross Douthat points out that's not good news for McCain.