Even so, the 61-seat GOP advantage in the House is substantial, and handicappers like the Cook Political Report and the Iowa Electronic Markets predict that Republicans will hold onto their majority.

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This unusual election year, however, raises another possibility: the very strategy that Republicans used to secure Congress could backfire. Their “great gerrymander” could become another “great dummymander.”

Winning by drawing marginal districts

After the 2010 Census, the Republican Party put in motion its plan to redraw congressional districts more favorable to conservative candidates. Whereas bipartisan gerrymandering creates safe districts for both parties, the GOP undertook partisan gerrymandering, which packs the other party’s voters into as few districts as possible and spreads out the gerrymandering party’s voters across many districts, each of which that party can win but often by uncomfortably narrow margins.

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Pennsylvania illustrates this strategy. In the 2012 election, Democratic congressional candidates won about 75,000 more votes than did Republican candidates, but the GOP captured 13 of 18 seats. Four of the five Democratic districts had been packed with Democratic voters. The safest of these districts scored D+38 on the Cook Report’s Partisan Voter Index (PVI), which means that voters in this district backed President Obama in 2008-2012 by 38 percentage points more than the national electorate.

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By contrast, the GOP currently holds four marginal districts, rated as R+2, R+1, or Even. Another six GOP districts are R+5 or R+6. The remaining three are R+9 or higher.

The Pennsylvania pattern holds up nationally, where the GOP holds numerous marginal districts. The chart below shows PVI ratings for all of the GOP’s House seats. Republicans hold 37 districts rated R+2 or lower and 18 at R+3 or R+4, for a total of 55 marginal districts. Democrats, by contrast, hold half as many.

Potential trouble for the GOP

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If the Trump collapse and Clinton surge continue, they could reveal the perils of partisan redistricting. That strategy created so many marginal Republican districts that if the GOP loses the bulk of the seats at or below R+2, it would also lose its congressional majority. A catastrophe that claimed every GOP seat at or below R+4 would bring the GOP caucus close to the size of today’s House Democrats.

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What Donald Trump is doing on the campaign trail share Share View Photos View Photos Next Image MANCHESTER, NH - NOVEMBER 7: Republican presidential candidate Donald Trump speaks during a campaign event at SNHU Arena in Manchester, NH on Monday November 07, 2016. (Jabin Botsford/The Washington Post)

More than that, many seats the Republicans lost would belong to newcomers, who include the most vocal Tea Party conservatives. Once again, this is an indirect result of gerrymandering, which typically ensures safer seats for the most senior party members.

In Pennsylvania, for example, three of the four marginal GOP districts are held by incumbents with less than five years of Congressional experience. By contrast, only one of the six most senior members of the Republican delegation has a seat with a PVI rating below R+6.

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Of course, Democrats shouldn’t be over-confident, even amidst a decisive Trump defeat. The influence of a presidential contest weakens as one travels down the ballot, and incumbents typically insulate themselves well enough to earn reelection rates north of 95 percent.

Moreover, cautious voters sometimes counterbalance one party’s Presidential victory by returning a congressional majority for the opposite party. Democrats may also have fielded too many subpar candidates in the marginal districts they didn’t imagine could be won.

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For these reasons, a Democratic House majority is still a long shot.

But 2016 is no ordinary election year. There’s still enough episodes left in this season for one more plot twist before Nov. 8. The final act might reveal the perils of the partisan gerrymander.

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