The answer is simple, and two-fold. First, John McCain has long outperformed generic Republicans, and is not tightly associated with their brand. Second, no one can unite all of the different Democratic voters on a national level… Had someone else other than John McCain been the Republican nominee, it is probably likely that virtually any Democratic nominee could have united all of the groups. Off-hand I can think of three solutions to this problem. First, lower-ticket Dems are actually going to have to help out the top of the ticket this time around, with Democrats in more Clinton-friendly areas campaigning with Obama (an with both of them campaigning as Democrats). Second, McCain needs to be defined as the conservative Republican that he is. Third, Obama needs to pick a good VP, someone who can bring more of the various Democratic groups together without emphasizing Obama’s weakness among some groups. I might be asking for too much on that one, a pick that is both reinforcing and balancing, but one can always hope.