there were apx 12 million votes cast in the dem primary in 2004, with the eventual winner, jacques kerry, getting about 10 million of them (he didn't really have any competition, admittedly). he eventually lost the election, winning 59 million votes.
this year, 24 million people have voted in the dem primary, and obama has garnered 12 million of those. 2004 and 2008 are completely different scenarios, and I would hesitate to try to extrapolate dem turnout in the general from the turnout in the primaries, but you really don't have to do more than glance at these numbers to realize that this is possibly real trouble.