"Most Americans agree that a president’s children should not be used as pawns in a political fight,” said Jay Carney, the White House press secretary. “But to go so far as to make the safety of the president’s children the subject of an attack ad is repugnant and cowardly.”   The National Rifle Association cast its opposition to Obama’s plans as “the fight of the century” in a letter circulated at the 35th annual Shooting, Hunting, Outdoor Trade Show and Conference, known in the trade as “the SHOT show,” in Las Vegas. “I warned you this day was coming and now it’s here,” Wayne LaPierre, the NRA’s fiery executive vice president, wrote in a fund-raising letter to the group’s 4 million members. “It’s not about protecting your children. It’s not about stopping crime. It’s about banning your guns … PERIOD!”