The U.S. Senate currently consists of 49 Democrats, 49 Republicans and 2 Independents. Both of those Independents caucus with the Democrats, although Senator Lieberman (I-Connecticut) often aligns himself with the Republican Caucus and has endorsed Republican Presidential Candidate Senator John McCain (R-Arizona). The other Independent Senator is Bernie Sanders (I-Vermont), who caucuses with the Democrats.
One would think that with an effective majority of 51-49 that the Democratic Senators would have an advantage but it hasn’t worked out that way. The minority party can "filibuster," forcing the majority party to need a 60 vote majority to pass a piece of legislation or even end debate.
The Republican minority in the Senate since the 2006 mid-term elections has used filibusters (or more often threats of them) to derail or delay legislation nearly 90 times.
There is a chance, however, that the 2008 elections will change the dynamic in Congress. According to Fivethirtyeight.com, an election projection website, Democrats are certain or likely to pick up five Senate seats: Virginia, New Mexico, Colorado, New Hampshire and Alaska. That would bring the Democratic majority to 56. Less likely pickups are in Oregon, North Carolina and Minnesota. 59.
Now on to the unlikely but possible. Mississippi, Georgia, and Kentucky, where the Senate Minority Leader Mitch McConnell finds himself up by only 2 points. 61 and a filibuster-proof majority.
If Senator Obama achieves a landslide victory, the chance of one or two of these races flipping to the Democrats becomes much more likely.