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 partys voters into as few districts as possible and spreads out the gerrymandering partys voters across many districts, each of which that party can win but often by uncomfortably narrow margins.
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 Reports 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.
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 GOPs 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.
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 todays House Democrats.
