Some theories of congressional politics suggest that Collins and Murkowski are in the sweet spot because they occupy coveted space in the political center. But moderates such as Dent, Collins and Murkowski are a rarity in Congress. Collins is considering leaving the Senate to run for governor of Maine in 2018, which would further increase the gulf between the two parties.

Why are moderates disappearing from Congress? My new book, “Opting Out of Congress: Partisan Polarization and the Decline of Moderate Candidates,” shows that both liberal Republicans and conservative Democrats get much less out of Congress than do their more ideologically extreme colleagues. That makes them less likely to run for Congress in the first place.

The political center has disappeared

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I created a data set of all Republican and Democratic candidates who ran for Congress from 1980 to 2012. I used new estimates of congressional candidate ideology to calculate the proportion of Republican candidates who were at least as liberal as Olympia Snowe, the veteran moderate senator and representative from Maine who retired in 2012, and the proportion of Democratic candidates who were at least as conservative as John Tanner, a longtime representative from Tennessee and founder of the moderate Blue Dog Coalition who retired in 2010.

Here’s what I found: The number of moderates who ran for Congress from 1980 to 2012 has clearly declined. Although moderate Democrats who resemble Tanner constituted more than 20 percent of Democratic candidates in 1980, they made up only 5 percent of the Democratic pool in 2012. Similarly, moderate Republicans who resemble Snowe made up 11 percent of Republican candidates in 1980 and a high of 16 percent of the pool in 1990, but had also dwindled to only 5 percent by 2012.

These trends are driving moderates away from Congress.

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Legislative service is far less rewarding for centrists than it was in the past

Although chances of winning have long been known to shape decisions to run for office, the non-electoral benefits of serving in office — lawmakers’ ability to affect policy, advance within the party, and forge bonds with their colleagues — matter, as well.

I spoke with more than 20 former moderate members of Congress. They described how, as the parties drifted apart, serving in Congress provided fewer benefits and became less pleasant for centrists.

First, their ability to influence policy outcomes has diminished. In the 1980s, moderates were, as one member noted, “oftentimes the difference on whether legislation would pass or fail.” But centrists’ bargaining position and policy impact waned as their numbers declined.

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Second, career ladders in Congress have become closed off to centrists. Obtaining a leadership position or even a desirable committee assignment became increasingly difficult for moderates. One moderate demoted to a less-prestigious committee said, “[Party leaders] can’t kill you, but what they can do is indicate, well, you’re done.”

Third, centrists have decried legislative service as “frustrating,” “unsatisfying” and “increasingly confrontational.” These day-to-day struggles wear on moderate members. As one put it, “Every day going in and being the odd man out. … It’s grueling, it’s exhausting; it’s corrosive.” The congressional environment has become an increasingly hostile place for those in the political center. As Charlie Dent put it last week, “increased polarization and ideological rigidity … leads to dysfunction, disorder and chaos.”

Moderate lawmakers are more likely to retire from office — and moderate potential candidates are less likely to run

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Moderates in the typical pipeline to Congress are far less likely to run for Congress than their more ideological colleagues. I analyzed the decision to run for Congress among 30,000 state legislators (since state legislatures are a common steppingstone to Congress), and I found that moderate state legislators who resemble Olympia Snowe and John Tanner are much less likely to run for Congress than are conservative Republican and liberal Democratic state legislators.

The difference is even more pronounced in open seats where state legislators are more likely to win because there is no incumbent in the race. The probability that a conservative Republican state legislator such as House Speaker Paul D. Ryan of Wisconsin runs for Congress when the seat is open is 20 times that of a moderate state legislator such as Snowe. The probability that a liberal Democratic state legislator such as House Minority Leader Nancy Pelosi of California will run is 25 times that of a moderate state legislator such as Tanner.

And it’s not that there are no moderates in state legislatures. Nearly 30 percent of Democratic state legislators in this sample are at least as conservative as John Tanner; nearly 20 percent of Republican state legislators are at least as liberal as Olympia Snowe. They just are not running for Congress.

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Surprisingly, moderates are even opting out of running in congressional districts where they would be especially likely to win. For example, we may think that a larger number of moderate Republicans would run in more liberal districts and a larger number of moderate Democrats would run in more conservative districts. Conversely, there may be fewer moderate Republicans and Democrats running in the most conservative and liberal districts, respectively.

But it turns out that very few Olympia Snowes and John Tanners are running for Congress, regardless of the makeup of the district or the makeup of district partisans.

The fact that moderates are not running for office across congressional districts suggests that there is more to running for Congress than winning elections. As moderate Republican Ileana Ros-Lehtinen of Florida said this year when she announced her retirement, “It was just a realization that I could keep getting elected — but it’s not about getting elected.”

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The result is a more polarized Congress

Congress’s sharp rise in partisan polarization has been discussed for the past decade. If the only individuals who seek congressional office come from the ideological extremes, polarization probably will continue. At the very least, bridging the gap between the parties may require finding new ways to stem moderates’ disinterest in running for and joining a polarized Congress.