Why this House race may be the last straw for Republicans in 2018

(CNN)House Republicans are currently holding onto the idea that they can hold their majority by the thinnest of threads.

A special election on Tuesday in Ohio may cut through that plan, setting off a sort of electoral panic with less than 100 days left before the midterm vote.
The race in question is in Ohio's 12th District. The resignation of Republican Rep. Pat Tiberi caused the special election. Both sides got the candidates they wanted: State Sen. Troy Balderson for Republicans and Franklin County Recorder Danny O'Connor for Democrats.
That's where -- in a normal election cycle with a national landscape generally even between the two parties -- the story would end. After all, this Columbus-area seat has been represented by a Republican for more than three decades straight. In the last round of redistricting, the seat was made more Republican to accommodate Tiberi, a close ally of then-Speaker John Boehner. Following that redraw, Tiberi won with 64%, 68% and 67% in his last three re-election races. Mitt Romney won the district over President Barack Obama by 10 points in 2012 while Donald Trump carried it by 11 in 2016.
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    But this is not a normal election cycle. Trump's unpopularity coupled with historic trends that work against the party in power in the midterms and a hugely energized Democratic base has turned the Ohio special into a dead heat, according to new polling from Monmouth University.
    In that survey, Balderson takes 44% to 43% for O'Connor, a statistical dead heat. That's concerning enough for Republicans. But the trend line of the race should be even more of a worry for GOPers. A month ago, in this same poll, Balderson led O'Connor by 10 points. That suggests momentum is on the Democratic side. Bigly.
    If there was any question of whether Republicans are nervous, Trump's decision to visit the district on Balderson's behalf this coming Saturday should clear that up. The President of the United States doesn't spend his weekend stumping in districts that the Republican candidate will win easily.
    What's important to remember here is that this isn't just about Ohio's 12th District. Not even close. This district -- and this race -- is a sort of guinea pig (or a canary in a coal mine) for both parties. And you can be sure that every Republican in the House -- and every Republican running for the House -- is watching the race closely. Will the attempt to link O'Connor to House Minority Leader Nancy Pelosi work? (It didn't when Republicans tried it against now Rep. Conor Lamb in a Pennsylvania special election earlier this year.) Can Trump drag Balderson across the finish line? And, most importantly, how big is the coming Democratic wave going to be? And how many Republican seats is it going to wash away?
    Here's one way to think about those last two, critical questions. According to the Cook Political Report's Partisan Voting Index -- essentially a way to compare the partisanship of every district to that of every other district -- Ohio's 12th is the 175th most Republican seat in the House. It has a PVI of R+7, meaning that, on average, it votes 7 points more Republican than a dead-even House district.
    Now, for a scary number for Republicans: There are 66 GOP-held seats that are as or less favorable to Republicans than Ohio's 12th. That's almost three times as many seats as Democrats need to retake the majority they lost in the 2010 election. (Democrats need to net 23 seats.)
    If Balderson comes up short on Tuesday, you can be sure every Republican running in those 66 districts will sleep very, very poorly. The excuses Republicans have offered over the past 18 months -- election is too far away for this result to be predictive, bad candidate, tough district, didn't raise enough money and on and on -- will no longer be operative. This Ohio seat is one a generic Republican should win. Balderson was an elected official who ran a credible campaign and raised the money he needed to.
    So, if Balderson loses, there will be only one logical conclusion to be drawn: The Democratic wave is real. And the follow to that: There may be nothing Republican candidates or party committees can do about it.
      That's a very scary prospect if you are a Republican on the ballot this fall. And a disheartening prospect if you are a major donor who is weighing whether to cut a big check to try to keep the Republican House majority.
      This close to an election, momentum matters a lot. A loss in Ohio on Tuesday would hand every bit of that momentum to Democrats. And without another competitive special election on the horizon between now and November 6, it's hard to see how Republicans could change the narrative if Balderson does come up short on Tuesday.