Edwards, being the most liberal candidate in the race, gets a lot of very liberal supporters interested in progressive change.  If he dropped out, most of those supporters would flow to Obama, as the Clintons don't have the best reputation among this crowd.

But those elites don't account for very many of John Edwards' voters.  And those folks are basically voting their demographics.  And the demographics of Edwards' supporters — older, white, more conservative — match the demographics of Clinton's supporters better. So if he were to drop out, most would probably switch to Clinton.  For that reason, folks thinking Edwards should leave the race and endorse Obama are probably barking up the wrong tree.  If you're an Obama supporter, the best thing Edwards can do is stay in and fight to the finish, depriving Clinton of votes.  If he exits, Clinton is likely to pick up many of those voters.