Updated at 5 p.m.: Revised to include details on a social media fight between Republicans and comedian Patton Oswalt, after an earlier update added comment from the Democratic Senatorial Campaign Committee and the Texas Republican Party.

WASHINGTON — Air Force veteran MJ Hegar of Round Rock on Tuesday announced a bid for the U.S. Senate, becoming the first major Democrat to launch a 2020 campaign in Texas against longtime GOP Sen. John Cornyn.

Hegar, who lost a congressional race last year, declared her decision on social media with a nearly four-minute video detailing her back story.

"Washington still has a lot of listening to do, and I'm going to make sure they hear us," she said. "Texas deserves a senator who represents our values: Strength. Courage. Independence. Putting Texas first."

Her announcement, while long expected, shakes up the political landscape in the Lone Star State, where Cornyn had been bracing for a serious challenge after Hegar, San Antonio Rep. Joaquin Castro and others expressed interest in taking him on.

Texas is now all but assured of being a bona fide battleground state next year.

Although a Democrat has not won a statewide race in Texas since 1994, former El Paso Rep. Beto O'Rourke — now a presidential candidate — last year signaled a change in the status quo by nearly pulling off an upset of Republican Sen. Ted Cruz.

Hegar, who alluded to O'Rourke's close call, appears eager for the fight, blistering Cornyn in her video as "that tall guy lurking behind Mitch McConnell in basically every single video."

"He calls himself Big John, but he shrinks out of the way while Mitch McConnell gets in the way of anything actually getting done in our government," she said, referring to the Senate Majority Leader. "Weakness, partisanship, gridlock — those are not Texas values, John."

Cornyn's campaign was quick to respond to Hegar's announcement with a flurry of jabs and fundraising solicitations.

Who got 65% of their donations from outside of Texas and was supported by Hollywood liberals like Rosie O’Donnell? TAKE THE QUIZ: — Team Cornyn (@TeamCornyn) April 23, 2019

The Republican's camp tweeted out an "Official MJ Hegar Quiz," asking questions like: "Who got 65% of their donations from outside of Texas and was supported by Hollywood liberals like Rosie O'Donnell?" They said Hegar is so liberal, she makes O'Rourke "look like nothing."

Cornyn campaign manager John Jackson also ripped Hegar as a radical lackey of Senate Minority Leader Chuck Schumer.

"MJ Hegar is Chuck Schumer's handpicked candidate for good reason: She supports government-run health care that would eliminate private insurance, crushing new taxes, and late-term abortion," he said, touting Cornyn's work to help sexual assault victims, veterans and others.

The National Republican Senatorial Committee, the GOP's Senate campaign arm, also piled on.

"New York liberal Chuck Schumer found his chosen candidate in MJ Hegar," said NRSC spokesman Jesse Hunt. "Hollywood Hegar's support for late-term abortion and government-run health care will play better with progressive Hollywood celebrities than with mainstream Texans."

As did the Texas Republican Party.

"The left continues to push further away from reality as they present voters a candidate handpicked by Democrat puppet master Chuck Schumer: California-supported Hollywood Hegar," said Texas GOP chairman James Dickey.

.@MJHegar tweaks @JohnCornyn for calling himself "Big John." #True. In 2008 ad, Cornyn wore a cowboy hat and fringed suede jacket atop a horse. Narrated by Sam Elliott sound-alike, with a spoof of Jimmy Dean song, "Big Bad John" & lots of terrible rhymes.https://t.co/cLW42a4ftr — Todd J. Gillman (@toddgillman) April 23, 2019

Hegar enters the Senate contest as something of a dark horse.

Cornyn, who served for years as the No. 2 Republican in the Senate, already has $7.4 million in his war chest. Hegar has about $37,000. Cornyn activated his campaign apparatus months ago, warning Republicans against complacency. Hegar will be building a statewide operation for the first time.

Then there remains the open question of whether Hegar will emerge from a Democratic primary unscathed — or at all.

Castro — whose twin brother, Julián, is running for president — said last month that he would "seriously consider running for Senate in 2020." He's a rising star in the House, where he holds key committee posts and serves as chairman of the Congressional Hispanic Caucus.

Joaquin Castro's spokesman, Matt Jones, said Hegar "brings a ton of great qualities to the race." But he also said "every candidate in this race has announced on their own timeline and is making their own case to the voters of Texas — Joaquin is no different."

"The last two campaigns he has fought, and won, started with a tough primary," Jones said, adding that the focus must remain on defeating President Donald Trump's "number one cheerleader and enabler in Texas — Washington John Cornyn."

Houston City Council member Amanda Edwards is also considering a Senate run, earning the attention of Cornyn's camp. Three lower-profile Democrats have also launched bids, per the Texas Tribune: Michael Cooper, Sema Hernandez and Adrian Ocegueda.

But Hegar, a combat veteran who helped blaze a path for women in the military, is no stranger to the underdog role.

She ran a shock campaign last year against longtime GOP Rep. John Carter, earning widespread interest for a slick video advertisement about her life and then coming within three points of toppling the incumbent in a district Trump won by more than 12 percentage points in 2016.

"Millions of views turned into an army of everyday people donating their time and giving what they could, and across the country people could see something special was happening in Texas," she said in Tuesday's video. "It was all so humbling for a working mom of two."

Hegar will still have to beat the rap that she couldn't even win her House race.

She took that criticism head-on in her launch video, spinning the loss by saying that "we didn't just close the gap in my district by almost 20 points — we helped change the status quo." She said her campaign helped motivate "new voices, new volunteers, new voters."

"And that made it all worthwhile," she said.

Hegar will also face persistent criticism from Cornyn and other Republicans that her campaign is fueled by Hollywood elites — a fact that was made abundantly clear after those Republicans seized upon her decision to include in her video a cameo by foul-mouthed comedian Patton Oswalt.

Cornyn's campaign dug up some of Oswalt's vulgar and sexually explicit tweets, while the Texas GOP called on Hegar to disavow the comedian's support.

"A person is known by the company they keep," said Dickey, the state GOP chairman.

Hegar, who had called Cornyn out of touch, didn't respond to those attacks. But others pointed out that Cornyn has backed Trump despite the president's history of bawdy remarks, and Oswalt on Tuesday seemed to delight in taunting Cornyn and other Republicans for being priggish.

"Oh my god you Pepperidge Farm [expletive]," Oswalt wrote to Cornyn on Twitter. "Go tell your constituents about the time you babysat Chester A. Arthur, drink your Ensure, and sit down."

In any case, Hegar will have help going forward, particularly if she secures the Democratic nomination.

The Texas Democratic Party, for instance, has already launched a multimillion-dollar "war room" to unseat Cornyn. The party's chairman, Gilberto Hinojosa, welcomed Hegar to the race, hailing her as an "American hero" whose "life has been about service to her country."

"The Texas Democratic Party is proud that strong candidates like MJ are running and we will be ready to support our nominees," he said. "Texas Democrats know that with hard work, strong values and bold action, victory is in our grasp."

The Democratic Senatorial Campaign Committee is also keeping close tabs on the race.

"Senator Cornyn is worried," DSCC spokeswoman Lauren Passalacqua said. "After nearly 20 years in Washington, taking orders from Mitch McConnell and voting to gut protections for Texans with pre-existing conditions, voters are ready for a new senator who will stand up for them."