There are three ways to get someone to do something: You persuade them, you offer them some benefit, or you threaten them with harm.

Leaving aside the last method (threats), so far inducing Hillary Clinton to drop out has focused mostly on persuasion. Yet the possibility of Barack Obama's paying Clinton's $20 million campaign debt (much of which is owed to herself) is one benefit that could be offered to Clinton — and has been broached.

The more obvious payoff would be to offer her the Vice Presidential slot on the ticket. If Obama really wants Clinton to end her campaign now, it is probably within his power to do so. But the price might be more than he wants to pay.