(CNN)Another one bites the dust.
On Monday, New Jersey Rep. Rodney Frelinghuysen, a noted moderate and chairman of the powerful Appropriations Committee, announced he was leaving Congress after 12 terms -- joining a rapidly expanding group of Republican moderates heading for the exits in advance of the 2018 election.
Frelinghuysen is the 35th House Republican to announce plans to leave by the end of the year -- as compared to just 15 Democrats. But, it's not just the raw numbers that matter here. It's who these retiring Republicans are -- and what the districts they represent look like.
Of the 23 districts Hillary Clinton won in 2016 that are currently represented by Republicans, six will be vacant in 2018. Reps. Ileana Ros-Lehtinen of Florida, Pat Meehan of Pennsylvania, Ed Royce and Darrell Issa of California and Dave Reichert of Washington are retiring while Rep. Martha McSally of Arizona is running for Senate.
Then there are the districts like the one Frelinghuysen is leaving in northern New Jersey where President Donald Trump beat Clinton by just a single point in 2016. Among those in similar seats are retiring Reps. Charlie Dent of Pennsylvania, Dave Trott of Michigan and Frank LoBiondo of New Jersey. Trump won them all but the underlying political realities of the districts suggest Democrats will make a major play to win them this fall.
The same was true for Frelinghuysen. After he faced minimal opposition for most of his time in office, Democrats -- enticed by Clinton's performance in 2016 -- recruited a top-tier candidate to challenge the incumbent. The nonpartisan Cook Political Report already rated the race a "toss up" prior to Frelinghusyen's retirement announcement.
What's even more interesting than the districts these members are leaving is the ideological profiles of the members themselves. They tend to hail from what Dent has described as the "governing wing" of the Republican party. They tend to be moderates or, at the least, less ideologically driven in their thinking. They see the work of government as necessary. They regard compromise as a success rather than a failure.
Frelinghuysen was -- and is -- a noted moderate and establishment figure. (Four Frelinghusens served as US senators and one, Theodore, was Henry Clay's vice presidential nominee in 1844.)
While Frelinghuysen was in his first term as chairman of the House Appropriations Committee, there was already some chatter among conservatives who wanted to challenge his chairmanship due to the fact that he voted against the Republican tax cut bill in late 2017. (Frelinghuysen said he voted "no" because of the cap on state and local tax deductions -- which will inordinately impact New Jersey.)
Frelinghuysen is in good company. While the Tuesday Group, the most prominent collection of House GOP moderates, doesn't release its membership, a number of House Republicans affiliated with the group are calling it quits including Ros-Lehtinen, Dent (a current chair of the group), Reichert, McSally and LoBiondo.
Dent explained his decision this way in his retirement announcement last fall:
"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."
Trump's victory in the Republican primary process in 2016 heralded a new day within the GOP -- a victory of hardliners over compromisers. The reverberations from that hostile takeover played out in elections in 2017: Ed Gillespie very nearly lost a Republican primary fight in the Virginia governor's race to an opponent who aligned himself closely to Trump while Roy Moore -- often described as "Trump before Trump" -- beat a sitting senator in Alabama. Both Gillespie and Moore ended up losing to Democrats.
Now the Trump effect is playing out in the composition of the Republican party in Congress. As I've written when Dent and Reichert and Royce and Issa retired, Frelinghuysen won't be the last member of the party's governing wing to walk away from Congress even with his party in power (and time left on his chairmanship). For moderates within the GOP, serving in Congress has stopped being fun.