Shankar Vedantam has an excellent Department of Human Behavior column in the Washington Post today, comparing the Democratic primary to a tragedy of the commons. A quote from political scientist Edella Schlager summarizes: "Rational individuals are trapped. To act rationally, to pursue one's self-interest, leads to collective ruin. To act irrationally, to place the collective interest above one's self-interest, exposes one to exploitation."
The article lists two solutions to the tragedy of the commons. The first is to create structures that reward behavior that favors the long-term welfare of the collective. The second is to have a regulatory body enforce limits. Vedantam suggests that the latter, having the Democratic party step in and call the game, is probably necessary to win the election.
The first solution, however, will keep more Democratic voters happy -- or less unhappy -- and motivated. And the way to do that is to link the two candidates' success. If both candidates commit that the ticket will be Clinton/Obama or Obama/Clinton, then the candidates have an incentive to campaign in a way that builds the Democratic party's likelihood of success come November.
Remember that Clinton offered Obama the VP post before she took the gloves off. Obama brushed her off, but the correct response would have been to offer her the VP post. By refusing to link their outcomes, Obama opened the door for the down-and-dirty contest that the primary has become.