As the Crystal Ball has noted repeatedly since early 2017, the Senate elections in the 2018 cycle feature two contrasting forces: highly polarized, partisan voting in elections running up against a tendency for non-presidential party incumbents to do well in midterms, even in states that backed the other party in the most recent presidential election. With 10 Democratic Senate incumbents running for reelection in states that President Donald Trump carried in 2016, the force that proves to be more unstoppable or immovable will be key to determining the majority in Congress’ upper chamber in November. After all, if voters hypothetically were to align their states’ Senate seats with their 2016 presidential party winner this fall, Democrats would lose 10 seats they currently hold while Republicans would lose just one, handing the GOP a net gain of nine seats. That theoretical result would give Republicans a 60-40 filibuster-proof edge.
Yet we know that such an outcome is highly unlikely at this point. While Republicans are favored to retain control of the chamber, and possibly gain seats in the process, Democrats do have a chance of winning a slim majority this November. To win 51 or 52 seats, Democrats would have to hold most or all of those 10 seats in Trump-won states while also adding one or two other seats in states the president carried — Arizona and Tennessee — while also winning the lone Clinton-won state the GOP is defending this cycle — Nevada. Although this is not a likely outcome, the fact we can credibly discuss it as a possible result is evidence that the historical trend of non-presidential party incumbents winning most midterm races may prove to be the stronger force once again in 2018….