"Vote for me you scmuck!"
What Athenae said, with one addition...
Some Democrats (let's call them The Joes) seem to think that the road to the White House is paved with the blood of their fellow Democrats. The Joes think they need to demean everyone else in their party in order to make themselves look better and more "electable". As Athenae so rightly points out, The Joes are asking Democrats to vote for the guy who is most able to humiliate them in public.
"Hello! I'm the guy who made everyone laugh at you at the prom! Vote for me!"
But what The Joes forget is that, even if they manage to get the party nomination, such tactics will undermine their chances in the general election. People are smart (generally). They know that when they vote for a President they aren't just voting for one guy. They are voting for an entire cadre of strategists, planners, thinkers and movers who will take over all the prominant positions in the Executive Branch. Hell, many people who were hesitant to vote for Bush in 2000 comforted themselves with the notion that he would be surrounded by "grownups" (guess that shows how not smart people can be).
So what does undermining his fellow Democrats do for The Joes when they win their party's nomination? It tells the general electorate, "Hey, you better not vote for me, because if you do you'll get all these other idiots as well."
Republicans win by convincing the voters that their "people" are at least as good, if not better, than the candidate. The Joes want to win by convincing the voters that they are better than their people even though it is their people who will ultimately run the country.
I guess we know how well that has worked.