Calculating Democrats' chances of regaining the Senate

I then ran the model with some different assumptions about where Obama’s job approval would likely be this fall. For example, let’s assume that the most likely outcome is that the president’s job approval will stay more or less around 45 percent, but acknowledge that it could be higher or lower. If we run 10,000 simulated races within these parameters, then the median outcome is no net change in the makeup of the Senate. Democrats win control of the Senate only about 5 percent of the time in this scenario.

On the other hand, if we see the president’s job approval increase to 50 percent (with allowances on either side) then the median likely outcome is a Democratic pickup of two seats, with Democrats winning the Senate 26 percent of the time.

If we see a substantial increase in the president’s job approval, to around 55 percent, Democrats win the Senate around 67 percent of the time, and pick up five seats. At 60 percent job approval, Democrats pick up eight seats, and control the Senate 94 percent of the time (this is different from what I found in 2014 because the model’s parameters have been re-estimated, there have been fewer Republican retirements than I anticipated, and problematic challengers, at least for now, haven’t really emerged).