As many as 1 in 5 seats in the House of Representatives may be competitive next year as population shifts and partisan realignment conspire to create one of the most widespread battlefields in generations.

Democrats will find themselves on defense in dozens of districts the party captured in 2018, including 31 districts President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE won in 2016. Already, the Democratic Congressional Campaign Committee has identified 36 members for its Frontline program, which protects endangered incumbents.

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Republicans, too, will have to defend districts in unexpected areas, seats that Democrats narrowly lost in 2018. Districts that have not been targets for years are suddenly in play, either because suburban voters revolted against Trump or because new residents are moving in and changing the makeup of those areas.

All told, 89 congressional races were decided by 10 percentage points or less in the 2018 midterm elections.

A shifting political battleground is common at the end of a decade, years after the decennial census reapportions districts and states redraw their political boundaries. New residents moving between states and across district lines distort old political balances, struck every 10 years to ensure districts have even population numbers.

Over the last seven years, however, those balances have become imbalanced at an unusually precipitous rate. Population shifts out of the Rust Belt and into the Sun Belt have created districts groaning with new residents.

Nearly two dozen congressional districts have seen their populations increase by more than 10 percent since the last census, according to the Census Bureau’s figures, and 52 districts have added at least 50,000 new residents.

In states where lawmakers carefully drew district boundaries to maximize their chances of winning, those population shifts become even more destabilizing.

Nowhere is that clearer than in Texas, where Republicans drew district lines that assured they would hold a supermajority of seats in Congress after the 2010 census. To do so, they had to draw several districts where Democrats would earn huge majorities and others where Republicans could win with 55 to 60 percent of the vote.

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“The GOP gerrymandering efforts rely on the most efficient distribution of GOP voters across as many districts as possible, the idea being to waste very few votes in each victory margin,” said Tom Bonier, a Democratic data analyst who runs the firm TargetSmart. “Combined with the population trends in many exurban areas, as they become more racially and ethnically diverse, the gerrymanders can become somewhat less effective the closer we get to the next decennial redistricting.”

Texas has grown at such a rapid pace that those careful calculations have been thrown out of whack. Where the average congressional district in the United States has grown by 23,200 people since the last Census, the average Texas district has grown by 74,000 people — and the average competitive Texas district, mostly centered around Houston, San Antonio, Austin and Dallas, has grown by an incredible 96,500 residents.

Texas Republicans publicly celebrate so many new residents, who come from states like California and New York in search of plentiful jobs and lower taxes. Privately, those Republicans bemoan the fact that the new residents still vote like they live in California or New York.

The phenomenon has occurred before: Nevada, New Mexico and Colorado, once swing states at the heart of the presidential battleground, have become increasingly blue as they attract new residents. Republicans have not won any of those states since George W. Bush won all three in 2004.

“We’ve seen this in places like Nevada before,” said Mark Gersh, a pioneer in Democratic data analytics. “So much of this is [the growing] minority population, and they’re more likely to vote than they were before.”

Democrats captured two House seats in Texas last year and a dozen seats in the state House. But several Republicans, including Reps. Michael McCaul Michael Thomas McCaulHouse passes legislation to crack down on business with companies that utilize China's forced labor House Republicans blame Chinese cover-up for coronavirus pandemic Engel subpoenas US global media chief Michael Pack MORE, Pete Olson Peter (Pete) Graham OlsonShakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' The time for HELP is now: Senate should pass bill to expedite recovery following natural disasters House Democrats reserve airtime for voters of color in Texas MORE, Will Hurd William Ballard HurdHillicon Valley: Oracle confirms deal with TikTok to be 'trusted technology provider' | QAnon spreads across globe, shadowing COVID-19 | VA hit by data breach impacting 46,000 veterans House approves bill to secure internet-connected federal devices against cyber threats House Democrats' campaign arm reserves .6M in ads in competitive districts MORE, Roger Williams John (Roger) Roger WilliamsThe Hill's 12:30 Report - Presented by Facebook - Yoho apologizes for accosting AOC Ocasio-Cortez accosted by GOP lawmaker over remarks: 'That kind of confrontation hasn't ever happened to me' Cook shifts 20 House districts toward Democrats MORE and John Carter John Rice CarterDonna Imam wins Democratic runoff to face Rep. John Carter House panel advances bill banning construction on bases with Confederate names Democrats see victory in Trump culture war MORE, won reelection by far narrower margins than they are used to, while freshman Reps. Dan Crenshaw Daniel CrenshawSecond night of GOP convention outdraws Democrats' event with 19.4 million viewers GOP sticks to convention message amid uproar over Blake shooting The Hill's Convention Report: Mike and Karen Pence set to headline third night of convention MORE, Ron Wright Ronald (Ron) WrightHispanic Caucus campaign arm endorses slate of non-Hispanic candidates Comedian Joel McHale: Reach out and help local restaurants, wear masks with your favorite message; Frontline Foods's Ryan Sarver says we are in inning 3 of the COVID-19 ballgame Cook shifts 20 House districts toward Democrats MORE and Chip RoyCharles (Chip) Eugene RoyPelosi must go — the House is in dire need of new leadership GOP lawmakers want answers from Disney on Mulan, China Freedom Caucus member Chip Roy touts bipartisanship in first campaign ad MORE won by slimmer margins than their Republican predecessors.

In many cases, the new residents moving into these new swing areas are from other parts of the country. Rep. Kenny Marchant Kenny Ewell MarchantHouse Ethics panel recommends ,000 fine for Rep. Schweikert's campaign finance violations Candace Valenzuela wins Texas runoff to replace retiring Rep. Marchant Ethics Committee reviewing Rep. Sanford Bishop's campaign spending MORE’s (R-Texas) district, which covers part of the Dallas-Fort Worth metroplex, includes about 800,000 people — almost 55 percent of whom were born in another state (30 percent) or another country (24 percent).

Marchant’s vote share has declined from 65 percent in 2014 to 56 percent in 2016 and just 50.6 percent in 2018. His campaign said it is counting on higher turnout among Republican voters who stayed home during the midterms.

“Rep. Marchant has been a dedicated, effective leader for the 24th district who has proven he knows how to get things done,” said Keats Norfleet, Marchant’s campaign spokesman. “This is a Congressional district that Republicans won by more than 6 points in 2016, and as a Presidential year, Republican voters will be more enthusiastic than ever to turn out and make their voices heard next year.”

Carter’s district, based in Austin’s fast-growing northern suburbs, is also emblematic of a changing state. He has won his last three elections with slimmer and slimmer support: He took 64 percent in 2014, 58 percent in 2016 and just 50.6 percent of the vote in 2018. Over that span, his district has added 133,000 new residents.

“It’s about a quarter of the electorate that’s brand new, first-time voters over the last two cycles,” said Jeff Burton, Carter’s campaign strategist. “The district’s changed considerably since 2012. It’s not as conservative.”

Carter spends every weekend in his district, in part introducing himself to those new voters. He has held five town halls since March, Burton said, and he prizes accessibility. He will begin holding office hours later this summer.

“We have a congressman in Judge Carter who works his butt off,” Burton said.

Olson’s district, south of Houston, is 171,000 residents larger than it was when the last census was taken, fueled by an influx of west Asian immigrants. As the district has become more diverse and better educated, Olson’s vote share has also shrunk, from 67 percent of the vote in 2014 to 51 percent last year.

“Republicans probably didn’t anticipate this happening in the suburbs. They may have known about the demographic shifts,” Gersh said. “Suburban voters in a place like Texas, who were at one time more supportive of Republicans, have deserted Trump.”