Sen. Arlen Specter(R-Pa.), a moderate who supports abortion rights, launched his long-shot bid for the1996 GOP presidential nominationby warning that Republicans would squander an historic chance to control both Congress and the White House if they abandoned the political center. "Let me say this as plainly as I can," Specter, 65, said in a speech at the Lincoln Memorial. "Neither this nation, nor this party, can afford a Republican candidate so captive to the demands of the intolerant right that we end up by re-electing a president of the incompetent left." Is he right? In an interview with TIME editors today,Massachusetts Gov. William Weld, a popular GOP social moderate, said that he himself declined to run in part because conservative Republicans swept last November's elections with the center's solid support.