Can the Republicans Hold the Senate in 2018?

Historical voting patterns and current economic and political conditions predict the Democrats can win 18 of the 33 Senate seats in contention this fall and take back control of that body.

The departure of Paul Ryan as Speaker provides yet another piece of evidence in favor of the Democrats taking back the House of Representatives this fall.  Faced with this prospect Senate Majority Leader Mitch McConnell and other Republican legislators and strategists argue that the party must now focus its efforts on maintaining its one-vote lead in the Senate.  In this essay I examine the prospects for a Democratic Senate victory as well.

At first glance, the Democrats’ prospects seem quite poor. The current “class” of Senators running for re-election last faced their voters in 2012 when Barack Obama was re-elected President. That helped the Democrats win or retain a number of seats in unlikely places like North Dakota, Missouri, and Montana. Twenty-three Democrats were elected to the Senate that year, compared to eight Republicans and two independents.  With so many more Democratic seats up in 2018, we might expect the GOP to maintain or even expand its slim Senate majority this fall.

However the Democrats themselves held a similar advantage in the 2016 election but failed to win back the Senate. The Republicans needed to defend the 24 seats they won during their “shellacking” of the Democrats in 2010, while just ten Democratic seats were at risk. Nevertheless the Democrats managed to swing only two seats into their column in 2016, suggesting that the relative number of seats at risk may not be a very powerful predictor of the eventual outcome.

One factor in the Democrats’ favor is the absence of Donald Trump at the top of the ballot come November.  Updating the chart from my previous article to include the 2016 election does not change this basic fact:

The Democrats did better than expected in the 2016 Senate elections, winning 53.7% of the popular vote.  That raised the average for the six “open seat” elections, when the President was not standing for re-election, to 48.8% from the figure of 47.9% I reported in 2016.  In general, though, having the President at the top of the ticket adds, on average, four percentage points to the vote for his co-partisans in the Senate. The Republicans are thus starting from behind despite the disparity in seats at risk.

Along with whether the President is standing for re-election, I identified three other factors that have systematically influenced the vote for Senate candidates since World War II:

  • the size of the previous popular vote for the current “class” of Senators facing re-election;
  • the President’s job approval ratings in polls near mid-term elections; and,
  • the year-on-year change in real disposable personal income per-capita.

That framework enables us to examine some possible scenarios for this fall.

There is some evidence that the size of the vote for a Senatorial class does influence how well that class fares six years later. The party advantaged in one election sees a drop in support when facing re-election.  In 2018 that factor helps the Republicans by about 1.8 percent, still not enough to overcome the deficit from not having the President at the top of the ticket.

As I find for House elections, a President’s “job-approval” rating influences the outcomes of Senatorial races.  A ten-point increase in the percent of Americans approving of the President’s performance results in a 1.2 percent increase in support for Senatorial candidates of the President’s party.

On the economic front, the Republicans have repeatedly touted their recent tax cut as providing an impetus to support for their party. As Bloomberg reported after a GOP retreat in February, “With President Donald Trump’s stubbornly low approval ratings and historical trends suggesting they’ll lose seats in the November mid-term elections, party leaders told lawmakers their salvation lies in hammering on the message that the tax cuts passed at the end of last year are putting more money in voters’ pockets.”

My earlier findings confirm that growth in real disposable personal income does have an effect on vote for the President’s co-partisans in the Senate.  In the simulations below, raising personal income growth by one percentage point yields an increase in the predicted vote for Republican candidates this fall by about 1.4 percent.

We can put these findings together and examine the model’s predictions for the Senatorial vote in 2018 given different combinations of presidential job-approval and growth in personal income.


or, visually,

In no likely scenario does my model predict a popular-vote majority for Republican Senate candidates this fall. Which of these scenarios might we see play out in November?

The job-approval figures I use in this analysis come from Gallup, since only that organization has published these ratings as far back as the 1940s.  Like most other pollsters Gallup has reported a slight rebound in Trump’s approval figures over the past couple of months, but they are still running around forty percent.

Gallup has begun reporting weekly ratings this year which explains the much lower variability of its 2018 results.  Also Gallup’s polling shows no systematic partisan bias in either direction but hews closely to the polling “consensus.”  However, Gallup may underestimate approval for President Trump by about 1.6 percentage points because it surveys all adults rather than just registered voters.

Personal income growth, despite the tax cuts, does not provide much reassurance for the Republicans either. Real disposable personal income per capita has shown only modest growth over the past few months, running at about 1.5 percent on an annualized basis.


While there has been a slight downtick since the January peak of 1.6 percent, it seems plausible that real personal income per-capita will show a gain in the neighborhood of 1.5 percent by November.

From the table of predicted vote outcomes, a job-approval rating of 40 and even two percent personal income growth corresponds to a 52-48 split in the popular vote favoring Senate Democrats this fall.  Because there is little “bias” in the translation from votes to seats in the Senate, winning 52 percent of the popular vote translates into a two or three seat margin this fall.


Using the equation shown in that chart, if the Republicans win 48 percent of the popular vote for Senate this fall, they should come away with only fifteen seats (45% of the 33 seats at risk).  A three-seat victory for the Democrats would flip control of the Senate even given the Vice President’s deciding vote.