Congress’s increasing political polarization owes more to demographic trends than it does to partisan gerrymandering of House districts, according to a new analysis this month that challenges the conventional wisdom on why Capitol Hill is so fractious.

The number of true “swing” districts has dropped more than 50 percent compared to 20 years ago, from 164 seats down to just 72 after the 2016 election, said the Cook Political Report.

But of the 92 swing districts that “vanished,” more than 80 percent of them disappeared because voters moved around, in a process demographers call “self-sorting” — not because lawmakers drew themselves safer districts, the analysis said.

“As it turns out, gerrymandering wasn’t as much of a factor in the House’s polarization as some redistricting reform advocates might argue,” the report’s authors said.

The median Democratic-held seat now tilts 14 points more toward the Democrats than the national average, which is double the 7-point difference from 1997, the report said. And the median GOP seat is now 11 points “more Republican” than the national average, compared to a 7-point difference 20 years ago.

Kyle Kondik, a political analyst with the University of Virginia’s Center for Politics, said that the power of incumbency and the geographic concentrations of Democrats in cities have also played significant factors in the current makeup of the U.S. House, which has been in relatively safe Republican control since 2011.

“Democrats end up wasting a lot of votes,” Mr. Kondik said. “Some of that has to do with redistricting, but it also speaks to the concentration of Democratic voters in tightly packed urban areas.”

Redistricting, which is the redrawing of congressional lines to account for population changes in the U.S. Census taken every 10 years, is controlled by lawmakers in most states.

The congressional map, which has streaks of blue along the coasts and in urban spots, with red dominating the interior of the country, shows just how ideology and geography have played out.

Some proponents of reforms say politicians tend to try to design their districts to be easily winnable — meaning voters of one party end up packed into a district, oftentimes leaving voters of the other major party packed into a neighboring district.

Those groups disputed the Cook findings.

“Extreme partisan redistricting” is indeed the driver behind the increasingly partisan districts, said Kathay Feng, the national redistricting director for Common Cause, an advocacy group that has pushed for reforms to the process.

She said that recent redistricting efforts have had a particularly strong effect because lawmakers have increasingly sophisticated technologies they can leverage to craft “enduring highly partisan districts.”

David Daley, a senior fellow with the group FairVote, said gerrymandering — the pejorative term used generally to refer to partisan-driven redistricting — and geography feed on each other.

“All are entwined, and all act as sort of accelerants on each other,” he said.

His group wants to see voters able to rank their choice of candidates in a process of gradual elimination, with the idea being that candidates who appeal to a broader swath of voters will ultimately make the final cut.

“You would elect a wider range ideologically,” he said. “The number of swing seats is so low right now that it has drained our politics of meaningful competition.”

Some reform advocates also pin a general rise in partisanship and gridlock in the country on redistricting, with the idea being that many incumbents now have more of an incentive to steel themselves against a primary challenge by protecting their left or right flank, rather than trying to appeal to the middle of the electorate.

“It’s not that everybody needs to move to the middle,” Ms. Feng said. “We have created a doughnut, and the middle has fallen out.”

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