The partisan playing field for the 2010 Senate elections is even, with the Republicans defending 18 seats and the Democrats also defending 18 -- including two in New York, which has both a special election and a regularly scheduled election, and the Pennsylvania seat gained by Sen. Arlen Specter's April 2009 switch to the Democratic Party. There was one special election this cycle, to fill the Massachusetts seat vacated by the death of Democratic Sen. Edward M. Kennedy. That race was won by Republican Scott Brown.

See our full chart of all Senate races by rating. Learn more about our race rating system.