The high failure rate of these revolts makes intuitive sense. Incumbent leaders tend to have strong ties to their colleagues, cultivated over many years through personal favors and party service.

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Furthermore, there is always the danger of retaliation: Even though ballots in leadership elections are secret, vengeful incumbents are usually quite skilled at figuring out who has voted against them. And Pelosi is reputed to lead with a strong hand.

Another way of estimating Ryan’s chances is to look at factors that predict vote choice in leadership elections. Our research has identified several key factors, including whether members are from the same state as a candidate and whether they have served on the same committees as the candidates, as well as the candidates’ gender, ideology and campaign contributions to colleagues. These factors suggest that Ryan is at a distinct disadvantage.

Ryan does serve on two major committees (Appropriations and Budget) with 26 other returning Democrats, a potentially rich reservoir of votes for the Ohioan. But nearly half of those Democrats are women, a key base of support for Pelosi, and Pelosi still has important ties to Appropriations, a committee on which she served prior to joining leadership. In addition, Pelosi can probably count of the votes of most, if not all, of the 37 Democrats from California, while Ryan is one of only four Democratic lawmakers who represent Ohio.

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Finally, Pelosi has been a far more generous contributor than Ryan. The most recent data from the Center for Responsive Politics show that, in the 2015-16 election cycle, Pelosi’s leadership PAC gave over $200,000 to 26 returning Democrats. By contrast, Ryan’s leadership PAC gave less than $5,000 to four Democrats who will serve in the 115th Congress.

Although Pelosi is the prohibitive favorite, her victory is not guaranteed. Ryan may benefit from other factors. For instance, we have found that seniority sometimes influences vote choice in leadership elections, and more junior Democrats (of which there are a growing number) may prefer Ryan over Pelosi.

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In addition, legislators who only narrowly won reelection may also be more inclined to support a change in party leadership, given the unexpectedly poor electoral performance of Democrats. Such was the case when Gerald Ford toppled Republican leader Charlie Halleck after the GOP’s disappointing showing in the 1964 elections.

More importantly, even if Pelosi wins, Ryan’s challenge could portend bigger problems for Pelosi and her fellow leaders in the future. Contested leadership elections can both reveal and widen cracks in support for the incumbent leader. For instance, in 1969 nearly a quarter of House Democrats voted for Morris Udall in his failed challenge against incumbent speaker John McCormack. A weakened McCormack announced his retirement from Congress a year later.

In short, if Ryan fails to defeat Pelosi but still wins a sizable number of votes, it could be an omen that the minority leader’s days in leadership are numbered.