Marc Ambinder walks through options, the most plausible seeming:

“I’m personally pro choice, and I’ve never made any bones about it. Having talked to constitutional scholars though and really having studied it more myself, I think Roe was terrible case law. Even liberal scholars will tell you that. And I’m a lawyer. All I’ve ever cared about is the construction. And having seen that, I really believe abortion should be a state issue, and if elected, I’m going to nominate people like Scalia and Roberts to the bench. Call it a litmus test or whatever you want, but I’m going to nominate judges who reflect my philosophy.”