* Republicans still favored to win House * Key races in both chambers too close to call * Biden's next two years at play By Nathan Layne and Tim Reid ALPHARETTA, Ga./PHOENIX, Ariz., Nov 9 (Reuters) - C ontrol 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.