Many of the most popular Democrats on Capitol Hill are progressives. So what happened to all the moderates?
Democratic leaders believe Republicans are exploiting the district-drawing process known as gerrymandering.
"I think the reason that we've lost some more moderate members of the Democratic caucus is due to really unfair Republican gerrymandering," the Democratic Congressional Campaign Committee Spokesperson Meredith Kelly tells Fox News.
As the DCCC tries to flip the House in 2018, Kelly believes Republican retirements give them an edge.
"There's a number of common themes with the types of people that are retiring," Kelly said of the seven Republicans who plan to retire. "I'm not willing to call them moderate, but they're at least people that are pragmatic and value compromise."
One of the Republicans who won't run again: Rep Charlie Dent, R-Pa.
Dent believes primary voters now value candidates friendly with the Commander in Chief over candidates eager to reach across the aisle.
"The litmus test prior to Donald Trump was purists vs. pragmatists," Dent said. "That was the litmus test. Now it's about loyalty to the President. That's what's changed."
The National Republican Campaign Committee chairman, Rep. Steve Stivers, R-Ohio, dismisses any talk about Republican infighting.
"Everybody works well and plays well together as a team," said Stivers.
Stivers is also downplaying the significance of Republican retirements, insisting most have been for "personal reasons".
The once powerful "Blue Dog Coalition," of moderate Democrats has seen their membership dwindle from several dozen, to just 18 members.
One member of the group in this Congress warns that extreme viewpoints are counterproductive.
"Every time you have extremes, you get pulled to the side and it prevents you from finding a solution," said Rep Josh Gottheimer, D-N.J.
But Gottheimer has a solution to get bills moving faster through the Congress: beer.
"We do a bunch of meetings with just 20, 30, 40 members in the room late at night and you know, someone gets some beer and tacos and everyone relaxes and just digs in on the issue," said Gottheimer. "I think that's actually how you're supposed to govern."