The playing field for the 2010 Senate elections is relatively even, with the Republicans defending 18 seats that will be on state ballots and the Democrats also defending 18, including two in New York, which has a special and a general election, and the seat recently gained by Sen. Arlen Specter's switch to the Democratic Party.

Massachusetts will hold a special election in early 2010, mandated by state law, to fill the vacancy caused by the death of Democratic Sen. Edward M. Kennedy.

CQ Politics rates two seats now held by Democrats and seven held by Republicans as "Tossups", or No Clear Favorite.