• Matthew Green

Will 2020 be a “wave” election?

Updated: Oct 15


Republicans are getting nervous. As Donald Trump’s chances of winning reelection dwindle, GOP lawmakers in the House and Senate have started distancing themselves from the president, hoping to avoid an electoral bloodbath in next month’s elections. Is a Democratic “blue wave” coming in November?


There are indications that Election Day will be good for congressional Democrats. Most predictive statistical models, such as those from FiveThirtyEight and the Economist, give the Democratic Party an all-but-certain chance of keeping the House and better than even odds of winning the Senate. Though these models are probabilistic and do not guarantee a particular outcome (see David Byler’s smart guide to how to interpret them), they do point to a favorable electoral environment for Democrats.


The same is true when looking at analyses of individual House and Senate seats. The Cook Political Report, for instance, has upgraded the chances of Democratic candidates in a number of races, such as the South Carolina Senate contest between GOP incumbent Lindsey Graham and challenger Jaime Harrison, which is now considered a toss-up.


Individual-level analyses like those of The Cook Political Report can also be fairly accurate predictors of how many seats a political party will gain in Congress. One way to test that accuracy is to compare the number of seats held by each party that are considered vulnerable, i.e. either toss-up seats or ones leaning or likely to flip to the other party. The difference between those two numbers suggest what the net partisan seat swing will be in the next election. So for instance, if there are twenty more vulnerable Republican House seats than Democratic ones, one should expect Democrats to gain twenty seats in the House, give or take.

As can be seen in the first chart, election results for the U.S. House closely match the number of Cook-estimated vulnerable seats (r=0.98). Looking at which party has more vulnerable House seats in September of an election year thus serves as a very good indicator of the number of seats that will actually switch party control.

The second chart shows that the correlation is less strong for Senate races (r=0.87). But this is not terribly surprising, since fewer seats are contested in any given election. That means an unexpected result in just a handful of races will significantly reduce the association between vulnerability and net partisan seat swing. (No green bars are visible for Senate races in 2006, 2010, and 2018 because net difference in vulnerable seats was zero.)


What does this tell us about Election Day 2020? Based on data from last month's Cook Report (shown at the far right of each chart), Republicans have a net four vulnerable House seats and a net four vulnerable Senate seats. If past results are any indication, this means that Democrats will improve their margin in the House slightly – and, more importantly, they could gain narrow control of the Senate.


Changes in seat vulnerability over time hint at the possibility that Democrats could do even better, at least in the House. The two graphs below show trends in net vulnerability based on quarterly Cook Report analyses, followed by the final election results.

For House races, when one party’s advantage in seat vulnerability increased over time, it ultimately did better on Election Day than suggested by September data. Given the House Democrats’ late-breaking momentum this year (as shown by the green line), the elections could yield the Party more than four additional seats in that chamber.

For Senate races, the 2013-14 election cycle -- the only one which resembled a real "wave" -- followed that pattern. The others did not, so it’s harder to make a claim that Senate Democrats will gain more than four seats in November. (Indeed, they could win fewer, as happened in 2016 and 2018, though recent developments suggest that's unlikely.)


While this is mostly good news for Democrats, it is hardly indicative of a so-called “wave election” in 2020. As I have noted elsewhere, many definitions of a wave election are bandied about, and most of them are pretty squishy. But the best empirically-based definitions, such as this one from Rob Oldham and Jacob Smith, would not consider an election in which one party picked up just four seats in each chamber to be a true wave.


Nor do other models suggest anything close to a blue wave is on the horizon. For instance, FiveThirtyEight predicts that the most likely election outcome is a five seat gain for House Democrats and a four seat gain for Senate Democrats, while the Economist estimates that Democrats will pick up eight seats in the House and five seats in the Senate. That’s miniscule compared to the huge partisan seat gains that have happened in the past, such as in 1974, 1994, or 2010.


Nonetheless, if Democrats keep their majority in the House and win the Senate and the White House, it won’t really matter whether the election is called a wave or not. With control of both the executive and legislative branches of the federal government, they will have the potential to profoundly shape domestic and foreign policy, shifting the country in a new direction for at least the next two years.

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