Tsahi Halyo

Democrats’ Chances are Overrated: The 2018 Midterm Elections

Democrats are favorites to win though a wave is far from certain

By: Tsahi Halyo

Democrats are favorites to win though a wave is far from certain

This coming November, voters across the country will have the chance to show their support (or anger) for the current White House administration through the ballot box. Given the Mueller investigation and the possibility of an open Supreme Court seat, control of Congress is paramount for both parties to achieve their political ends. With such a tense situation, one could reasonably assume that most voters would turn up on election day and vote. Unfortunately that isn’t the case – on average, only 39% of eligible voters come out to vote on midterm election years. This represents a marked difference to the presidential elections when the average turnout rate is 56%, nearly 1.5 times greater than the midterm turnout. However, despite this trend, the President’s low approval rating means that many assume that there will be a large Democratic wave in the 2018 midterm that will flip the House and perhaps the Senate. Unlike cable news pundits, let’s actually examine some of the claims that can possibly affect the Democrats’ chances of retaking Congress. These include: the claim that midterms usually go against the party controlling the White House; that the GOP has a midterm turnout advantage and that the generic ballot accurately predicts midterm outcomes.

Preliminarily, for the purposes of establishing a baseline, it’s always worth looking at online betting markets in which people can gamble on the results of future elections to see what people think the odds for a Democrats flipping control of a house of Congress are. Presently, the betting market  gives Democrats 2:1 odds of retaking the House while only giving them 1:2 odds of retaking the Senate. For some context, consider that Intrade, a popular betting market, gave the GOP a 55% chance of winning the House in 2010 and a different betting market gave Hillary a 89% chance of winning the presidential election on the night before the election. Thus, it is wise to take betting odds with a grain of salt. The difference in the odds given for the House and Senate races is grounded in the difficult Senate map this year in which Democrats will have to defend 26 seats while the GOP will only have to defend 9 (and, as we know from past elections, the incumbent normally has an advantage).

Now, let’s examine the first claim: midterms usually go against the party in control of the White house. Of the 18 midterms since the end of World War II only two have gone in favor of the president’s party and both of those elections occurred in unique circumstances. The first was in 1998 after the House unpopularly impeached Clinton and the second was in 2002 after 9/11 when the president’s approval rating was greater than 70%. On the other hand, the usual result of midterm elections is a loss for the president’s party. As seen in figure 1 below, the loss of seats can range anywhere between 4 seats and 63 seats (the 2010 midterm really was a disaster for democrats). On average though, the president’s party loses 25 seats in the House in a midterm election. The House has 435 seats meaning that a party needs to control at least 218 of them to control the House. Presently, republicans control 238 seats; if they lost the average 25 seats in the midterms, their 238 seat majority would be reduced to a 213 seat minority. This obviously portends ominous news for the GOP retaining their 8 year House majority, and is in line with the odds given by betting markets.

Figure 1: Midterm Election Swings with Regard to the President’s Party Affiliation

The second claim argues that the GOP has a midterm turnout advantage. As it turns out, Republicans are more likely to vote in a midterm election than Democrats. In races since 1978 the GOP has on average had a 3% midterm turnout advantage. That is, the GOP did 3% better with midterm voters compared to their margin among registered voters. A closer look at the data reveals an insight in line with our first claim. Under a Democrat president, the mean Republican turnout advantage was 5% irrespective of the Democratic president’s national popularity whereas under a Republican president the mean turnout advantage was a paltry 1%. And, when their president was unpopular with an approval rating below 40%, as was the case with George Bush in 2006, they had no turnout advantage. Clearly the magnitude of their turnout advantage is correlated with their party being out of the White House and the popularity of the incumbent president. Given that Donald Trump’s popularity is on par with Bush’s in 2006 (they both have a ~40% approval rating) it’s quite possible that the Republican turnout advantage won’t materialize.

The third claim concerns the accuracy of the generic ballot in predicting the midterm margin of victory (as a percent of the vote, not in seats) this far out from the election. The generic ballot is based on polls that ask respondents to name which party they plan on voting for in the coming midterm election. The polls are averaged (it’s a weighted average) to produce a smoother prediction. Historically, predictions made by the generic ballot a year out from the midterm elections were highly correlated with the results of that year’s midterms. When considering the cases of every midterm since World War II, the correlation is r = 0.9 (if r = 1, then the predictions would be perfectly accurate; 0<| r |<1), an extraordinarily high correlation.

Figure 2: Correlation Between the Generic Ballot Margin and the Midterm Margin

As is seen in figure 2 above, the relation between the generic ballot margin and midterm results margin is linear. It’s trend is approximately –Picture4

The current generic ballot margin is approximately 7% in favor of the Democrats. The general trend would suggest that the GOP is going to lose the midterms popular vote by a margin of 8.5%. How that will translate into a loss of seats is a difficult question to answer. If Democrats win every House seat won by a Republican in 2016 by a margin of less than 8.5% then they’re only going to win ~15 seats, too few to retake the House. If they win every district that Clinton lost by a margin of less than 8.5% then they’re on track to win ~30 seats and flip the House. The reality will likely be an outcome in between those too.

In light of the above, it’s clear that Democrats are the favorites to win the House though their victory isn’t assured. It’s still possible, albeit unlikely, that the GOP will narrowly hang on to their majority. Also, given that the GOP controls the White House, Democratic gains in Congress are to be expected, though their magnitude may be shy of securing a majority in the House and almost certainly too small to secure the Senate. Due to Trump’s unpopularity, Republicans can’t hope rely on their midterm turnout advantage to stem their losses as, historically unpopular GOP incumbents negate their midterm advantage. The generic ballot is usually a good bellwether for ascertaining the magnitude of an electoral wave. However, in this case, the range of likely outcomes can allow either party to control the House in 2019, with Democrats as slight favorites; Republicans have much to fear but, Democrats shouldn’t be too arrogant – the election could still hold surprises for both sides.


Sides, J. (2014, November 03). The 2014 midterm election fundamentals (in 4 graphs). Retrieved April 15, 2018, from https://www.washingtonpost.com/news/monkey-cage/wp/2014/11/03/the-2014-midterm-election-fundamentals-in-4-graphs/?utm_term=.0323b3760fc4

F. (2018, January 09). Do Republicans Really Have A Big Turnout Advantage In Midterms? Retrieved April 15, 2018, from https://fivethirtyeight.com/features/do-republicans-really-have-a-big-turnout-advantage-in-midterms/

Enten, H. (2017, November 10). There’s No Reason To Think Republicans Will Be In Better Shape A Year From Now. Retrieved from http://fivethirtyeight.com/features/theres-no-reason-to-think-republicans-will-be-in-better-shape-a-year-from-now/

Wall Street Journal. (n.d.). Retrieved from https://si.wsj.net/public/resources/images/OG-BE845_201803_TOP_20180305181721.jpgg

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