As the United States heads toward the midterms, an extraordinary number of incumbents are facing primary challengers from the ideological extremes. That trend has been increasing for years, and may reach a high point this year. From the 2000s to the 2010s, serious primary challenges tripled in number. Similarly, I find that extreme candidates tripled their average share of the vote in those primaries. Moreover, since the Obama administration, most primary challengers have come from the ideological extremes and received support from activist groups like Justice Democrats on the left or the Club for Growth on the right.
In my recent research I examined whether this insurgence of ideological primaries has pushed congressional Democrats and Republicans even further apart.
In at least one way, the answer is yes. Members of Congress who’ve been “primaried” from their party’s ideological extreme are less likely to co-sponsor legislation with someone from the other party. In fact, I find that about one-fourth of the increase in polarization in Congress over the last four decades can be attributed to the increased threat of being “primaried.”
Kurt Schrader (D-Ore.) and primary challengers from the left
Consider, for instance, how Rep. Kurt Schrader (D-Ore.) reacted to being primaried in 2014.
In this May’s Oregon primary, Schrader lost his party’s primary to a more ideologically extreme challenger. But 2022 was not the first time Schrader was challenged from the left.
In 2010 and 2012, Schrader won the Democratic primary unchallenged. After that, Schrader was challenged from the left in every single election year, and fought off two relatively competitive challengers before his loss this year. In 2014, a political newcomer attacked Schrader for working with Republicans to draft legislation that increased the amount of land for timber harvesting. In 2016, Schrader was challenged by a former state House member who aligned himself with Sen. Bernie Sanders (I-Vt.) and said he was running “[b]ecause a lot of Democratic voters want real fundamental change, a political revolution, and they’re not willing to settle for politics as usual.” In 2018, Schrader faced a weak challenge from an inexperienced candidate, again from the left. And in 2020, he faced a more competitive challenge from a candidate who compared himself ideologically to Rep. Alexandria Ocasio-Cortez (D-N.Y.).
Did the recurring primary challenges change Schrader’s behavior in office? According to the Lugar Center, in the 113th Congress (2013-14), which began before his first primary challenge, Schrader was the 10th most bipartisan legislator of the 435-member House of Representatives. But over the next several years, Schrader became less and less likely to co-sponsor bills with Republicans. In the 114th Congress (2015-16), he was the 50th most bipartisan representative; 59th in the 115th Congress (2017-18); and by the 116th (2019-20) and 117th (2021-22) Congresses, he ranked in the high 90s.
But only a fraction of Congress faces a primary challenge in any given election
So each primary challenge significantly changes that particular legislator’s approach to governing. But such challenges aren’t that common, so those individual shifts don’t widen Congress’s overall divisions very much. Even though competitive primary challenges are more common than in the past, only about 60 to 70 members are primaried in a given cycle.
So does the increasing threat of a primary challenge from the ideological fringe shape other legislators’ behavior?
Measuring the primary threat — and how it affects legislators
To answer this question, I developed a technique to measure the threat of an ideological primary challenge. I defined threat as the share of the vote that an ideological challenger could be expected to win in a given primary election. I constructed the equivalent of a forecasting model to estimate that share of the vote by modeling past outcomes.
My model assumes that the sense of threat, within a given party, depends on how well ideological challengers perform overall in that particular election cycle and how well they perform in similar districts. Schrader’s tough primary challenge from the left in 2020 sent a clear signal to Rep. Peter A. DeFazio, another moderate Democrat in a very similar Oregon district. Since DeFazio’s district is geographically, politically and economically very similar to Schrader’s, I used the proportion of the vote that Schrader’s actual primary challenger won to model the estimated vote share a liberal challenger could have won in DeFazio’s district.
Next, I evaluated the effects of those potential ideological challengers on how often incumbents co-sponsored bills with members of the other party.
By analyzing the behavior of 1,360 legislators from 1980 to 2016, I found that the increase in competitive ideological primary challenges explains about one-quarter of the drop in bipartisan bill cosponsorship since the 1980s. That effect is nearly equal in each party.
Just one more factor in the polarized U.S. Congress
To be sure, partisan primaries are not behind all the polarization of the last several decades. As other research has shown, Congress’s extreme division today also results from geographic and partisan “sorting,” that is the phenomena of increased ideological uniformity within the parties and within places across the country; the rise of negative partisanship, or siding with a party because of extreme dislike for the other side; and party leaders’ increasingly common strategy of undermining the opposing party to win or maintain power. But the fear of being primaried is clearly part of the equation.
Richard C. Barton is the Democracy Fellow at Unite America and a PhD student in government at Cornell University, and will be an assistant teaching professor of policy studies at Syracuse University beginning in the fall of 2022. You can find him on Twitter at RichardBarton85.
Updated Oct. 5, 2023.