Texas Democrats turned out in force ahead of the first-in-the-nation primary Tuesday in what could be an early hint of a midterm election backlash against President Trump, but their party remains a long shot to make much of a dent in Republican political dominance of the state.

Democratic early voting across Texas' 15 most-populous counties, the only figures available, more than doubled that of the last non-presidential cycle in 2014, while the number of Republican early ballots cast increased only slightly. Total Democratic early votes exceeded Republican ones roughly 465,000 to 420,000, though those figures combined accounted for less than 9 percent of the state's total registered voters.

Democrats haven't won any of Texas' 29 statewide offices since 1994, the nation's longest losing streak. That's expected to continue this cycle despite any possible "Trump effect" because Democrats fielded little-known candidates against top Republicans such as Gov. Greg Abbott and Lt. Gov. Dan Patrick. Even Attorney General Ken Paxton, who has been indicted on felony securities fraud charges, remains favored for re-election.

A record six Texas Republicans and two Democrats are leaving Congress, meaning the state will be losing clout on key House committees. But none of those open seats are expected to flip. They've drawn so many hopefuls from each party, that most primary races won't have anyone winning a majority of Tuesday's votes, meaning runoff elections May 22 will determine who will be on November's general election ballot.

Democrats have a better shot in November of unseating three Republican congressional incumbents – Rep. Pete Sessions in Dallas, Rep. John Culberson in Houston and Rep. Will Hurd in a district stretching hundreds of miles from San Antonio to El Paso. Hillary Clinton beat Trump in all three districts in 2016, but primary runoffs are likely in each of those races.

One of the Democrats leaving his House seat, former punk rock guitarist Beto O'Rourke, has generated national buzz in his uphill bid against Republican US Sen. Ted Cruz. Neither faced serious primary challengers but Mr. O'Rourke has out-raised Senator Cruz and the incumbent has warned conservatives against complacency, suggesting that liberals will "crawl over broken glass in November to vote," against Mr. Trump and the GOP.

The Democrats have had their own internal strife in Texas over congressional hopeful Laura Moser, who moved from Washington to her native Houston to try and unseat Representative Culberson. The Democratic Congressional Campaign Committee, fearing Ms. Moser may be too liberal to win the general election, blistered her for comments from a 2014 Washingtonian magazine article in which Moser said she'd "rather have her teeth pulled out" than live in rural Paris, Texas. Strategists will be watching if she advances to a runoff despite attacks from fellow Democrats.

Despite that, Texas Democratic Party chairman Gilberto Hinojosa has remained optimistic, noting that Trump beat Mrs. Clinton by fewer than 10 percentage points in Texas in 2016, the smallest margin of victory for a Republican White House candidate since 1996.

"Texas is the fastest growing state in the country, we're getting younger and increasingly diverse," Mr. Hinojosa said. "These demographic shifts are a positive trend line for a big-tent progressive political party."

Republican political consultant Derek Ryan noted that only about 3 percent of those casting ballots early in the Democratic primary were first-time voters, meaning most Texans participating "were probably voting Democrat in general elections in previous cycles."

"Three percent, that could make a difference in some smaller races, but in a statewide election I don't think that's enough to sway anything," Mr. Ryan said. "Democrats are showing up in the primary election, does that mean more are going to show up in the general election?"

A close Republican primary race Tuesday could be for land commissioner, where George P. Bush was the first member of his family to win his first election four years ago but drew an unlikely challenger in Jerry Patterson, a former Bush supporter who preceded him as land commissioner.

Another key contest is the Democratic gubernatorial primary, where the top two contenders in a crowded field are former Dallas County Sherriff Lupe Valdez, backed by the party's establishment, and Andrew White, who opposes abortion and whose father, Mark, was governor in the 1980s. Neither Mr. White nor Mr. Valdez may win a majority of Tuesday's votes, though.

Governor Abbott has an eye-popping $43 million in campaign cash, tops among gubernatorial hopefuls nationwide, and isn't expected to be seriously challenged by any Democrat. Instead, he's focused on attempting to unseat members of his own party, endorsing the Republican primary challengers to three state House incumbents who backed past ethics reform measures that might have limited gubernatorial power. That includes state Rep. Sarah Davis, a suburban Houston Republican who supports abortion rights.

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Representative Davis counters that her district's residents "will not be told for whom to vote."

This story was reported by The Associated Press.