WASHINGTON — Federal regulators are pledging rigorous reviews of Comcast Corp.'s proposed purchase of NBC Universal to ensure that it would not stifle competition or harm consumers. Christine Varney, the Justice Department's antitrust chief, and Julius Genachowski, chairman of the Federal Communications Commission, offered no indication at a hearing Thursday of what the outcome of those reviews could be. But many lawmakers and industry analysts expect regulators to approve the deal with conditions to prevent a combined company from abusing its market power. At Thursday's Senate Commerce Committee hearing, lawmakers expressed concern about the dangers of allowing the nation's biggest cable TV and broadband company to take control of NBC's vast media empire. "When consolidation occurs in these markets, we need to pay attention," said Commerce Committee Chairman John D.