WASHINGTON – The retirement of John Paul Stevens, the Supreme Court's leading liberal but a justice who also could find conservative allies, will set off an election-year political battle over President Barack Obama's second high court pick.

Stevens said Friday he will step down when the court finishes its work for the summer in late June or early July. He said he hopes his successor will be confirmed "well in advance of the commencement of the court's next term." Obama hailed Stevens as an "impartial guardian of the law" and promised to move quickly to nominate a replacement....