Donald J. Trump, girding for a long battle over presidential delegates and a potential floor fight at the Cleveland convention, has enlisted the veteran Republican strategist Paul J. Manafort to lead his delegate-corralling efforts, according to people briefed on Mr. Trump’s plans. Mr. Trump confirmed the hire in a brief telephone interview. “Yes,” he said, “it is true.” Mr. Manafort, 66, is among the few political hands in either party with direct experience managing nomination fights: As a young Republican operative, he helped manage the 1976 convention floor for Gerald Ford in his showdown with Ronald Reagan, the last time...