From The New York Times' Nate Silver:

The table below contains the FiveThirtyEight estimates of the probability of victory for each party in the 33 Senate seats to be contested this November. These forecasts, although they are informed by polling and other objective measures, are ultimately reflections of my best judgments and also account for “intangible” factors like candidate quality and the partisan orientation of the state. (We traditionally switch over to purely tangible factors in the summer, once polling becomes more robust and we post the official forecast model for the Senate.)

Still, if the elections were held today, the Senate races would be more localized than they were in 2006, 2008 or 2010, and Mr. King’s decision and the outcome of the presidential race could swing the partisan balance.