The NYTimes has a very nifty senate calculator
http://www.nytimes.com/packages/html/politics/2004_ELECTIONGUIDE_GRAPHIC/ and after playing around, it seems very difficult to win back the senate. The democrats would have to win just about every one of the competetive senate races (and that's not including SC as a competetive race in the first place and assuming the republicans will pick this up).
Now, I hope I'm wrong and I would like to see the dems pick up every one of the races that are considered competetive, but there are so many dem senators retiring and many from relatively conservative states. There's no given that the dems will keep LA and FL, and there's no guarantee that they can beat Lisa Murkowski of AK, even though her dad appointed her to the seat (considering AK is such a red state).
While I'm extremely pessimistic about getting the majority in the senate, I do believe it's very possible and likely Kerry will win the presidency. The House is also unlikely, but with a Kerry victory and some coattails in these key battleground states, we may eek out a majority.
I wanted to make it clear to some that were bitching about Kerry and claiming they would vote in other races, but not the presidential -- THIS IS THE MOST COMPETETIVE OF THE THREE FEDERAL RACES. It is true that Kerry may have a somewhat difficult time with a republican senate, but little if anything at all can be accomplished, even if democrats claim a slim majority in the senate or the house, if the presidency is occupied by Bush.
So don't be stupid -- if you are concerned the direction this country is taking, don't not vote or make a protest vote -- actually vote for the only way something can be accomplished -- Vote for John Kerry.