BEDFORD, Texas — Deep in the suburbs northeast of Fort Worth, Democrats trying to win the Texas House for the first time in years have been getting help from a surprising source.
For 16 years, until he left office in 2013, Todd A. Smith was a Republican representing these suburbs in the Texas House of Representatives. His district covered a fast-growing hub of middle-class and affluent communities next door to Dallas-Fort Worth International Airport.
When it came time to decide whom he would support for his old seat, Mr. Smith said he had no hesitation — he threw his endorsement to the Democrat in the race, Jeff Whitfield.
“This is no longer my Republican Party,” Mr. Smith said last week while sitting outside his house, which has a “Republicans For Biden 2020” sign on the front lawn.
“This is the Trump party,” he said. “If you give me a reasonable Republican and a crazy Democrat, then I will still vote for the Republican. But if you give me a lunatic Republican and a reasonable Democrat, then I’m going to vote for the Democrat, and that applies in the presidential race, and it applies in the Whitfield race.”
After a generation under unified Republican control, Texas is a battleground at every level of government this year. President Trump and Senator John Cornyn are fighting for their political lives, and five Republican-held congressional seats are in danger of flipping.
But some of the most consequential political battles in Texas are taking place across two dozen contested races for the Texas State House, which Republicans have controlled since 2003. To win a majority, Democrats must flip nine of the chamber’s 150 seats — the same number of Republican-held districts Beto O’Rourke carried during his 2018 Senate race, when he was the first Texas Democrat to make a competitive run for Senate or governor in a generation.
Mr. O’Rourke has organized nightly online phone banks that are making about three million phone calls a week to voters during the campaign’s final stretch. His organization helped register about 200,000 Texas Democratic voters in an attempt to finish a political transformation of Texas that began with his Senate race.
“I actually won more state House districts than Ted Cruz,” Mr. O’Rourke said in an interview last week. “It’s just that the candidates in nine of those, the Democratic candidates, didn’t end up winning.”
Control of the Texas House comes with huge implications beyond the state’s borders. A Democratic state House majority in Texas would give the party one lever of power in the 2021 redistricting process, when the state is expected to receive as many as three new seats in Congress. It would also give them a voice in drawing Texas state legislative lines for the next decade.
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Officials from both parties said the difference between the current unified Republican control of the Texas state government and Democrats controlling the state House could be as many as five congressional seats when new maps are drawn.
“Flipping the Texas House this year can be the key that unlocks a Democratic future in Texas,” said John Bisognano, the executive director of the National Democratic Redistricting Committee. “With fair maps, Democrats will be able to compete all over the state and build a deep bench of candidates who can run and win statewide.”
Nowhere in the country has there been a surge of voting to match the one in Texas. Through two weeks of in-person early voting, more than 6.9 million Texans have voted — a figure that accounts for more than three-quarters of the entire 2016 turnout.
The turnout is highest in the state’s biggest metropolitan areas, which are the core state House battlegrounds — and are six of the 10 fastest-growing counties in the country. There are five competitive state House seats in Tarrant County, which includes Fort Worth, five more in other Dallas suburbs, and eight in greater Houston.
“I’ve always been political my whole life,” said Gina Hinojosa, a state representative from Austin whose father is the chairman of the Texas Democratic Party. “Now, suddenly, everybody is so political. The last election has had the result of engaging everyday people in our political process.”
Texas Republicans have sought to tie Democrats running for the state House, who are campaigning on issues like health care and increasing school funding, to the most liberal proposals in their party. Gov. Greg Abbott on Thursday launched a digital advertisement attacking Mr. O’Rourke’s past statements on police funding, gun control, tax policy and the Green New Deal.
This week, the governor and other Republicans jumped on former Vice President Joseph R. Biden Jr.’s pledge during the presidential debate on Thursday to “transition away from the oil industry,” a bedrock of the Texas economy, saying that such a move would cost the state hundreds of thousands of jobs and shrink revenues that pay for schools.
“He is an albatross around the neck of down-ballot candidates in Texas,” said Jared Woodfill, a Houston conservative activist and lawyer who is a former chairman of the Harris County Republican Party. “Biden just lost Texas.”
Democrats said they were not worried, calling the outcry over Mr. Biden’s remarks an attempt to distract voters from more pressing issues, including the continued spread of the coronavirus in Texas.
Suburban voters do not appear to be buying Republican arguments during the Trump era that Democrats will turn their communities socialist. Polling in 10 targeted Texas state House districts shows Mr. Biden gaining an average of 8.6 percentage points, while Democratic state House candidates have gained 6.5 points since March in surveys conducted by the National Democratic Redistricting Committee, which has invested more than $1 million in Texas over the last two years.
The suburban voters of 2020, said Steve Munisteri, a former Republican Party of Texas chairman who worked in Mr. Trump’s White House, have far more in common with urbanites than they do with the more conservative voters who used to populate the outer edges of Texas metropolitan areas.
“Because of urban growth, many of what are considered traditional suburbs in Texas metropolitan areas really are just extensions of the urban areas,” Mr. Munisteri said.
Collin County, a suburban area 20 miles north of Dallas, has two competitive state House districts that Mr. O’Rourke carried in 2018. In six years, the county has added 200,000 people. It now has a population of more than 1 million people and has gone from a Democratic wasteland to one teeming with liberal volunteers.
In 2014, when John Shanks moved to Collin County, there were about 20 dedicated Democratic Party volunteers. Now Mr. Shanks, the executive director of the county’s Democratic Party, has several hundred — so many that he has trouble finding work for them all.
“We’ve had about four years of people getting used to the idea that their vote really can matter,” Mr. Shanks said. “We’ve grown into realizing that you can make a difference. And as they realize that and wake up, things become more competitive.”
Bedford sits in a part of the Dallas-Fort Worth region that has been deeply conservative for decades. Republicans have held the region’s state House seat since 1985, and the Northeast Tarrant Tea Party was one of the most influential Tea Party groups during the Obama era.
The outgoing state representative, Jonathan Stickland, is a bearded Cruz-style firebrand who supported gun rights and wore his .40-caliber semiautomatic pistol at the Texas Capitol. In 2015, The Texas Tribune called him the “chamber’s antagonist-in-chief.”
Mr. Stickland apologized in 2016 after an online posting he made in 2008, before he ran for elected office, was unearthed by a political opponent. In the posting on a fantasy football site, he responded to a man’s request for sex advice by writing: “Rape is non existent in marriage, take what you want my friend!”
Yet after years of sending conservatives to Austin, the district has changed. In just two years, the Republican advantage shrunk from 9,100 votes for Mr. Trump in 2016 to 1,167 when Senator Ted Cruz defeated Mr. O’Rourke in 2018.
“When you’re hearing people who’ve spent a lifetime voting Republican and they say, ‘The party has left me,’ I don’t know that we’ve ever heard that before,” Mr. Whitfield, the Democratic state House candidate, said as he stood in a parking lot outside the Bedford Public Library, an early-voting site.
Steps away in the same parking lot, Mr. Whitfield’s Republican opponent, Jeff Cason, disputed any notion of a widespread Republican defection.
“I’m a man of faith, and I just believe the doors are opening for us, and if the Lord wants us in Austin, we’ll be there,” Mr. Cason said. “I’m not getting any sense of Republicans leaving our camp.”
Julie McCarty, who was the president of the Northeast Tarrant Tea Party and is now the chief executive of the group it transformed into, the True Texas Project, attributed the Democratic gains in the region to Republicans not being conservative enough.
“Republicans want to be left alone. We want smaller government. When we can’t get that, we move where we can,” she said. “Therein lies the answer to what causes Tarrant to turn purple.”
For Mr. Smith, the former Republican legislator, 2020 has been a year to split his ballot. In addition to the Biden sign and his support for Mr. Whitfield, he has a yard sign for Jane Nelson, a Republican state senator running for re-election. And he voted for Senator John Cornyn, the Trump ally locked in a tough re-election fight with M.J. Hegar, a Democrat and former Air Force helicopter pilot. Years ago, Mr. Smith threw Mr. Cornyn a fund-raiser at his house.
“I have mixed feelings about it,” he said of his vote for Mr. Cornyn. “But I trust what I believe to be his honest convictions.”