"Candidates don't have any legal right to possess that money at all," said Jan Witold Baran, a partner at Washington D.C.-based law firm Wiley Rein. The leftover money goes to the individual in charge of the super PAC, typically the treasurer, who is identified in the committee's FEC filing.
As Baran notes, the head of the PAC can do any number of things with the leftover cash. "The purpose of the PAC may change but they can continue operating as long as they have money," he said. "They may support the GOP nominee in the general election." Karl Rove's super PAC American Crossroads, for instance, supports a range of political candidates and issues.
But contributing to other campaigns isn't the only thing a super PAC is free to do. Unlike campaign money for presidential, senatorial or congressional campaigns, super PACs are allowed to use the leftover funds for personal expenditures, said Michael Toner, another partner at Wiley Rein. "The FEC has a prohibition on personal expenditures for campaign funds but it does not extend to super PACs," he said. "I could accept a yacht from a super PAC, for instance." He noted that the expenditures would still be taxable. And if yachts aren't your thing, super PACs can also donate the remaining money to charity.