 By naming her VP, you're kind of naming someone as a successor. And also just from a campaign finance perspective, she's the only candidate who could come in and inherit his entire campaign war chest. The president can direct his campaign to transfer some of the $90 million-plus that they have to the DNC, but they also have to pay off debts. They have to potentially offer refunds to donors. They can't just give it to any other candidate who comes into the picture unless that candidate is vice president.