270toWin has launched an interactive map to help you follow the 2008 Senate races. There are currently 49 Democrats and 49 Republicans in the Senate, with 2 Independents caucusing with the Democrats, giving that party majority control.  In 2008, 35 seats are up for election, in 33 states.   Of these 35 seats, 23 are currently held by Republicans, 12 by Democrats.

Current polling by the Cook Political Report indicates that it is likely the Democrats will retain control of the Senate, and may even gain a few seats.   At this point, however, it doesn’t look like they will reach the 60 seats needed for a supermajority.

60 votes are needed in the Senate to end debate or filibuster on a bill via a cloture vote.   (From senate.gov:  Cloture is the only procedure by which the Senate can vote to place a time limit on consideration of a bill or other matter, and thereby overcome a filibuster).    Therefore, a party controlling 60 seats (and who can get a party-line cloture vote) has dominant control of the legislative agenda, as it can bring any issue to the full Senate for a vote.