As should be apparent from the new chart that appeared on the site in the last few moments, our Senate tracking is ready to go. We currently project the Democrats to gain 6.7 seats. However, the distribution is rather asymmetric: four current GOP-held seats (VA, NM, NH, CO) are exceptionally likely to go to the Democrats, two others (AK and MS-B) are more or less toss-ups (we project the Republican as a very slight favorite in both), and then there is a "long tail" of seats that the Democrats may have an outside shot at. Among Democratic-held seats, meanwhile, only Louisiana looks to be under any threat; we give the GOP about a 20 percent chance of winning it.
We give the Democrats about a 20 percent chance of reaching 60 seats counting Joe Lieberman and Bernie Sanders, and a 13 percent chance counting Sanders but not Lieberman. The Democrats are nearly certain to hold on to an outright majority of seats, whether or not Lieberman's seat is counted.
There is, obviously, a lot of explanation required for what goes into all of this, which will come in doses over the next day or two.