Gerrymandering happens when partisan lawmakers draw legislative districts in a way to disadvantage political opponents, either by packing them all up in a small number of districts to contain their influence, or splitting them across multiple districts to dilute their political clout. To see how it works in practice, look no farther than North Carolina and Pennsylvania, where after the 2010 Census Republicans drew maps to give themselves huge political advantages in U.S. House races.

Just how lopsided? In 2016, Democrats in North Carolina won 47 percent of the statewide popular House vote but took home 23 percent of the seats. Pennsylvania Democrats, meanwhile, won 48 percent of the statewide vote in contested House elections but 27 percent of the seats. Big differences between popular votes and seat totals are one of the telltale signs of a heavily gerrymandered state.

But the two states' paths diverged after 2016. North Carolina held on to its gerrymandered districts for the 2018 election, just barely, despite multiple court rulings decreeing that the maps are unconstitutional. In Pennsylvania, on the other hand, the state Supreme Court redrew the maps earlier this year, in time for the election, creating a much more competitive electoral landscape.

The difference in the outcome at the ballot box is striking.

North Carolina, where the old maps were still in place, got an electoral result in 2018 that’s virtually identical to that of 2016. Despite a Democratic wave in which more than half the state’s voters opted for a Democratic House candidate, Democrats won one-quarter of the contested seats.

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In Pennsylvania, on the other hand, a 53 percent majority in the popular vote yielded a hair under half of the contested seats — a big difference from 2016, when 48 percent of the vote gave Democrats 27 percent of the seats.

I’m using the word “contested” deliberately — in a small number of races in these states in these years, one of the major parties didn’t field a candidate. I’ve excluded those races from this analysis, since they result in ballots cast for only one political party.

I’ve focused on these two states because they were frequently rated as the most gerrymandered states in the nation on a number of different measures, and because they saw intense litigation and public debate over gerrymandering this year. They both happen to be GOP gerrymanders, although it’s worth pointing out that Democrats have a record of doing this, too — Maryland being the most noteworthy example.

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Considering all seats in these two states, contested and otherwise, Pennsylvania’s House delegation moved from 11 Republicans and seven Democrats before the election to a 9-9 split after, a result that’s much more in line with the statewide popular vote than in previous years.

In North Carolina, however, the results are unchanged: Democrats held three seats before the midterms, and they’ll hold three seats afterward, despite increasing their state popular vote share in contested elections by a full 5 percentage points.

In these abstract discussions of seats and vote tallies it’s easy to lose track of what this means from a governance perspective. Judge James A. Wynn Jr. of the U.S. Court of Appeals for the 4th Circuit offered some big-picture perspective in a ruling against the constitutionality of North Carolina’s gerrymandered map earlier this year. He lamented that voters in the state “have been deprived of a constitutional congressional districting plan — and, therefore, constitutional representation in Congress.”

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Proper representation in Congress is one of the cornerstones of U.S. democracy — the whole system is built upon it. Gerrymandering gnaws away at that representation, making it difficult for voters to elect a government that represents their will.