Obama administration appointees have blocked several big mergers in recent years. But some observers, particularly on the left, believe the government still has not done enough to truly reinvigorate antitrust enforcement. Sen. Elizabeth Warren (D., Mass.) gave a prominent voice to those views Wednesday, saying corporate concentration in many sectors of the economy was harming innovation, small businesses and the middle class. “Today in America, competition is dying,” the senator said at an event hosted by the New America think tank. Ms. Warren said antitrust agencies have approved too many problematic mergers by companies that promise asset sales or other concessions to preserve competition. Too often, she said, such “fashionable” steps do not fully ensure competition and sometimes they don’t work at all, creating higher prices and reduced services for the public.