By Nathan Layne and Tim Reid ALPHARETTA, Ga./PHOENIX, Ariz. (Reuters) -Control of the U.S. Senate hung in the balance on Wednesday as Republicans moved closer to securing a House majority, a day after Democrats outperformed expectations and history in U.S. midterm elections. The Senate contests in Nevada and Arizona, where Democratic incumbents were seeking to hold off Republican challengers, were not yet called, with thousands of ballots still to be counted. If the parties split those races, the Senate’s fate would come down to a Georgia runoff election for the second time in two years, after…