In a year when the Decomcratic Party has been handed the Presidency on a platter, only the Dems could throw it out the window.  New polling indicates that enough Clinton supportes and Obama supporters will not support the others' candidate in the GE.  While it is generally accepted that these polling numbers won't stick come November, the numbers are so significant that enough could stick to sway the outcome to a Republican win.  And this polling has been consistently high during this primary season.
Harry Reid and other party leaders have stated a Democratic candidate will be determined in June.  Maybe there will be a Super Delegate convention where all will have to state their support.  Whatever the method, June is good.  That leaves plenty of time for a GE campaign.
Except, supporters of the candidate who has not won the nomination may very well sit out the GE or vote for McCain.  In a + or - 2% or 3% polling differential between all three candiates that we see today, Democrats sitting out the election or voting for McCain ensures a Republican presidency in November. 
IMO, the polarizing attacks by the Clinton camp and the Obama camp, Penn and Axelrod, Kos and MyDD, you and me, have resulted in many of us drawing a line in the sand.  Where has that gotten us?  A McCain presidency?
I support a Democratic GE nominee consisting of both Democratic Primary candidates on the ticket.  The fight wil lbe who's on top of the ticket.  The fight won't be to request Democtatic Party voters to come back.