COLUMBUS — The Republicans running for Georgia’s open U.S. Senate seat took aim at each other’s records and experience in a debate Saturday, with the three congressmen in the race facing criticism over their time in Washington. The Republican primary fight has been closely watched nationally, with Republicans needing six seats to claim a majority in the U.S. Senate. Republicans can’t afford to lose the seat, which opened when Republican Sen. Saxby Chambliss announced plans to retire.