The Senate and House races are usually undercard matches in presidential years. But 2012 may be different. Republicans are expected to hold the House, and are probably slight favorites to win the White House at this point.......
If Democrats are going to keep a toehold in Washington, their only hope is retaining the Senate....
This is especially true because much of what the Republican Congress will probably focus on in 2013 is spending cuts, which can -- and almost certainly will -- be done through budget reconciliation (which only requires a majority vote).
But the Democrats’ chances of holding the Senate aren’t particularly good. Right now Republicans hold 47 of the 100 seats; if they win the presidency, they would need to pick up three more seats to have a majority. As of today, they are favored to pick up two Democratic seats. Two Republican seats are tossups, along with six Democratic seats. If we assume Democrats pick off one of the two Republican tossups, and that Republican pick off half of the Democratic tossups, Republicans would have a 51-49 majority.
http://www.realclearpolitics.com/articles/2011/09/28/odds_favor_gop_gaining_senate_control_in_2012_111492.html
