As political nail-biters go, the most interesting battle in the 2012 election does not involve anyone named Obama or Romney. While the general election is going to be decided in six or seven swing states that have a reasonable likelihood of going for either presidential candidate, control of the Senate rests in a much more complex tangle of eight or nine states. Right now, our model of polls, prediction markets and historical data suggests that Democrats have a 48.7 percent likelihood of retaining control of the upper body.
The Democratic Party currently has 53 senators in its caucus to the Republican Party's 47 senators. But the way the cards fell with the 33 seats up for re-election this year was not kind to the incumbent party. Democrats are defending 23 seats while the Republicans are defending only 10 seats in this year's election. Stated another way, the Democrats have just 30 seats confirmed for the next session to the Republicans' 37 seats among the 67 total that aren't up for re-election for two or four more years.
Since the vice president casts a tie-breaking vote in the Senate, the Democrats must win 20 seats and the presidency or at least 21 seats to retain the Senate. The Republicans must win 13 seats and the presidency or at least 14 seats to take over the Senate.