Washington (CNN) The surprise resignation of Ohio Rep. Pat Tiberi on Thursday marks the latest example of GOP members affiliated with the so-called "governing wing" of the Republican Party calling it quits even as their side controls both chambers of Congress and the White House.

Tiberi is the 18th House Republican to announce plans to leave Congress sometime between now and January 2019 . More importantly, he joins a rapidly swelling list of House GOPers with influential committee assignments, close ties to the party leadership and pragmatic approaches to governing to walk away. In recent months, Reps. Ileana Ros Lehtinen (Florida), Dave Trott (Michigan), Dave Reichert (Washington) and Charlie Dent (Pennsylvania) have all decided to leave. On the Senate side, Sen. Bob Corker of Tennessee, another member of the pragmatic establishment within the Republican conference, announced his plans to retire earlier this month

In every Congress, retirements happen. There are 535 members of Congress and, in any given two-year stretch, a decent chunk will decide to do something else with their lives. (Tiberi is leaving to take a job running the Ohio Business Roundtable .) As "Inside Elections" editor Nathan Gonzales noted last month, the average number of House members retiring without seeking any other office is 22. That number -- including Tiberi -- is just 12 right now.

The issue isn't the raw number of retirements. It's who is retiring.

Take Tiberi. He is close to House Speaker Paul Ryan. He is the fourth-ranking member on the powerful ways and means committee and chairman of the health subcommittee. He would be a strong favorite to win again in 2018 with his massive $6.6 million cash on hand.

For someone with that profile to leave Congress when his party holds the majority -- and there is a major tax reform/tax cut measure in the offing -- would have been unthinkable even a few years ago. (Ways and means is the primary committee expected to deal with the tax legislation.)

Why is it happening then -- not just with Tiberi but with several other GOP members with similar profiles?

Rep. Charlie Dent gave us a big hint in announcing his own plans to leave. Here's part of what he said:

"I have done my best to make a meaningful, positive impact. As a member of the governing wing of the Republican Party, I've worked to instill stability, certainty and predictability in Washington. I've fought to fulfill the basic functions of government, like keeping the lights on and preventing default.

"Regrettably, that has not been easy given the disruptive outside influences that profit from increased polarization and ideological rigidity that leads to dysfunction, disorder and chaos."

Short version: This isn't fun anymore.

What the 2016 Republican presidential primary proved -- over and over again -- is that the sort of paint-within-the-boundaries conservatism favored by Jeb Bush, Marco Rubio, John Kasich and almost all of the other people that Donald Trump beat isn't what the GOP base wants. They want confrontation. They want controversy. They want people who view compromise -- of any sort -- as capitulation.

The 2017 version of the Republican Party is a whole hell of a lot closer to Steve Bannon than Mitch McConnell. (Bannon is putting that theory to the test by trying to recruit serious primary challengers to all but one of the Senate Republicans seeking re-election in 2018.)

That makes people like Dent, Corker and Tiberi dinosaurs in Trump's Washington.

They might not lose in a primary fight next year. Republicans may well hold onto their majorities in the House and Senate in the 2018 election. But their kind is dying off.

Pragmatists are less and less common not just in Congress but in the country, according to a fascinating Pew study on our increasing polarization . In 1994, the median Democrat was more conservative than 30% of Republicans. In 2017, the media Democrat is more conservative than just 3% of Republicans.

Politicians are reactive to the public. If voters reward ideology and combativeness, that is what politicians will give them. (Trump's 2016 campaign is the ur-example of that phenomenon.)

But if rank ideological tribalism isn't really your thing, Washington isn't a terribly fun place to be these days. My guess? There will be more members with the profile of Tiberi deciding that 2018 is the last year they want to spend in Congress. And Republicans will have no one to blame for those retirements but themselves.