(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.

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