Forget Kansas. This chart shows why Republicans need to worry about 2018

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(CNN)Political Washington is in the midst of a heated debate over what -- if anything -- the somewhat-closer-than-expected special election in Kansas on Tuesday means for next November's midterm elections.

No matter where you come down on that question -- I think there were clear warning signs for Republicans in their victory -- the history of the first midterm election of a newly-elected president should scare Republicans tasked with retaining their House majority in 2018.
This chart, which comes via the highly-sought-after quarterly PowerPoint presentation from Republican lobbyist Bruce Mehlman, is one Republicans should save and remind themselves of every day between now and next November.
The president's party usually takes a beating in midterm elections
If past is prologue, then Mehlman's chart suggests that Republicans will face across-the-board losses in 2018 and could even lose control of the US House.
    The chart documents seat losses in the House, Senate, state legislatures and governor's mansions in the first midterm election of the last eight presidents -- from John F. Kennedy in 1962 to Barack Obama in 2010.
    The average loss in the House for the President's party over that period is 23 seats. If you take out the 2002 midterms -- a totally unique situation created by the terrorist attacks of 2001 -- the average loss is 26 seats. If Democrats make gains consistent with that history, they will be right on the edge of re-taking the House; the party needs a 24-seat gain for the majority in 2018.
    It's worth noting that more recent first term, midterm elections have produced larger seat swings than the average. In 2010, Obama lost 63 seats -- and control of the House. In 1994, Bill Clinton lost 54 -- and control of the House. Trump's approval rating, which sits in the low 40s, according to Gallup's daily tracking poll, is also lower than where Obama and Clinton stood at this time in their respective terms.
    All of which should worry Republicans more than anything that happened in Kansas on Tuesday night.
    The history of Senate seat switches in a president's first midterm election is slightly less conclusive and, given the 2018 map, may be less predictive as well.
    Since 1962, the average loss for the President's party in the Senate is 2.5 seats -- although three presidents (Kennedy, Richard Nixon and George W. Bush) actually picked up Senate seats in their first midterms.
    Another factor working in Senate Republicans' favor is the map -- in which 25 Democratic seats are up in 2018 as compared to just eight for the GOP. And it's not just the raw numbers. Hillary Clinton won only one of the eight states where a Republican is up next November. By contrast, Trump won 10 of the 25 states where Democrats are up in 2018 -- including five (Indiana, Missouri, Montana, North Dakota and West Virginia) -- where the Republican nominee won by double digits.
    As for governorships and state legislatures, Republicans are nearly maxed out in terms of gains -- meaning that the party is primed to suffer large-scale losses consistent with historical norms. (The President's party has typically lost five governor's mansions and 245 state legislative seats in the first midterm.)
    Add it all up and you can see why 2018 should be a tough year for Republicans. Of course, if Trump's candidacy (and victory) proved anything, it's that history is only predictive until it isn't anymore.