WASHINGTON — President Barack Obama nominated appeals court judge Merrick Garland to the Supreme Court on Wednesday, thrusting a respected moderate jurist and former prosecutor into the center of an election-year clash over the future of the nation's highest court. Obama cast the 63-year-old Garland as "a serious man and an exemplary judge" deserving of a full hearing and a Senate confirmation vote, despite Republican vows to deny him both. Standing in the White House Rose Garden with Garland, Obama argued the integrity of the court was at stake and appealed to the Senate to "play it straight" in filling the seat left vacant by the death of Justice Antonin Scalia.