The simulation has proved rather accurate over the past few cycles. In 2014, it suggested that if Barack Obama’s job approval were 44 percent on Election Day, Democrats would be expected to lose nine seats. This is what happened. In 2016, it suggested that Democrats should gain three seats; they gained two. In 2018 it suggested that Democrats should lose two seats with President Trump’s job approval at 45% (as it was on Election Day, according to exit polls); Democrats lost two…

That leaves us with presidential job approval as the major open question. As of this writing, Trump’s is hovering around 42.5 percent in the RCP Average. If we assume that Trump’s job approval holds at this point on Election Day, we would expect Republicans to lose two seats, with 95 percent of the simulations falling between a loss of four seats and no net loss. Republicans lose three or more seats (enough to lose the Senate, assuming they also lose the presidency) in about 40 percent of the simulations. So, we would say that Republicans are favored to hold the Senate, but are far from overwhelming favorites to do so.

Looking at individual seats, we see that Republicans would not be favored to be competitive in races for any Democratic-held seats except Alabama’s, which reflects conventional wisdom. If the remaining seats did not flip in the good GOP year of 2014, it is difficult to see them flipping if a Republican president’s job approval is at 42.5 percent.