ATLANTA — Businessman David Perdue has kept Georgia's open U.S. Senate seat in GOP hands, beating his Democratic challenger on the strength of white voters in the reliably Republican state. Democrats hoped Michelle Nunn, the daughter of a popular champion of bipartisanship, former Sen. Sam Nunn, would enable them to pick up a seat in an otherwise dismal midterm election. But Perdue overcame repeated attacks on his business record by arguing that Nunn would be would be a rubber stamp for President Barack Obama.