Republicans have a retirement problem. And it's only going to get worse.

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Washington (CNN)In the last 10 days, four Republican House members have announced they won't be running again in 2018.

One of those four was Rep. Frank LoBiondo, who leaves behind a very competitive district in southern New Jersey.
"As some of my closest colleagues have also come to realize, those of us who came to Congress to change Washington for the better through good governance are now the outliers," LoBiondo said in his retirement statement. "In legislating, we previously fought against allowing the perfect to become the enemy of the good. Today, a vocal and obstinate minority within both parties has hijacked good legislation in pursuit of no legislation."
LoBiondo, in short, wasn't having fun anymore. The Congress -- and its agenda -- is being wagged by the tail of the dog, aka the 40 (or so) most conservative members who oppose any measures that seek to build compromise with the other side. As a result, nothing gets done. And people who came to Washington to get something done get frustrated and leave.
    Now consider what happened on Tuesday night across the country. Republican candidates were swamped -- particularly in the suburbs -- by Democrats thanks, in large part, to President Donald Trump's dismal poll numbers. Democrats turned out in large numbers to send a message to Trump while Republicans, broadly speaking, were less enthused to go vote.
    No fun. Not getting anything done. Energized Democratic base. Unenthused Republican base.
    That's a toxic mix if you are a Republican member of Congress -- especially one in a competitive district or someone who has been around Washington long enough to have watched the idea of legislating slowly but surely die.
    Given the political environment inside and outside of Washington, it doesn't take a genius to imagine that there are going to be more retirements from the Republican side before 2018 is all said and done.
    And, already, the number of GOP members leaving office in 2018 or having already left in 2017 is running ahead of historic norms -- as this amazing chart by CNN's Sam Petulla shows.
    The short version of that chart is that two-and-a-half times more Republicans are retiring than Democrats, as of today. And that the rate of Republican retirements at this point in the election cycle is higher than it has been in recent elections.
    What's also critically important is not just the raw number of retirements but the kinds of seats that are being left behind. And, in that regard, too, things look bad for Republicans -- with the likelihood they will get worse.
    In addition to LoBiondo, Reps. Ileana Ros-Lehtinen of Florida, Dave Reichert of Washington, Charlie Dent of Pennsylvania and Dave Trott of Michigan have also announced they won't run for re-election in 2018. All hail from districts that are either competitive or potentially competitive. Hillary Clinton carried Ros-Lehtinen and Reichert's districts. Trump carried LoBiondo's seat by four points. Same with Trott's district.
    All of those members just retired. They didn't decide to run for another office or get some sort of cushy, high-paying job. They just left. And, in the case of LoBiondo and Dent, we know their decisions were heavily influence by the current tribalism in American politics. We know because they said so.
    And each of their districts will be targets for Democrats in search of the 24 seats they need to regain the majority in Congress.
    History suggests that open seats are easier to win than seats in which an incumbent is seeking re-election. It stands to reason, then, that the more retirements, the more danger for Republicans -- and the more margin for error in Democrats' effort to retake the House.
    History also provides a very daunting fact about incumbents running for re-election when the president is in their party -- and unpopular. Since 1946, the average seat loss in the House in a midterm election when the party's president is under 50% is 36 seats. That's the average!
    If you are a Republican House member today, that's a very sobering set of facts. And it's not all that hard to see why retiring as a winner rather than spending the next year of your life slogging through a nasty race you still might lose might start to look very appealing.