Cisneros conceded Wednesday morning but struck a defiant tone, saying her race was proof "a brown girl from the border with a whole community behind her could take on the machine."

"This is just the beginning," she said in a statement. "The first thing we had to defeat was the culture of fear — and our movement was victorious in proving we're within striking distance of bringing fundamental change to South Texas.”

Throughout the campaign, Cisneros knocked Cuellar as “Trump’s favorite Democrat” and made a generational argument against the eight-term incumbent. Taking on Cueller, who opposes abortion rights and once had an A-rating from the National Rifle Association, Cisneros quickly became a cause celebre for the left.

She championed both "Medicare for All" and the Green New Deal and notched endorsements from Sen. Bernie Sanders (I-Vt.), Elizabeth Warren (D-Mass.) and Ocasio-Cortez.

"As someone who was also outspent 10 to 1, she can absolutely pull it off. But if she doesn't pull it off, I don't think we've seen the last of her," Ocasio-Cortez said in an interview shortly before the election.

The high-profile endorsements gave Cisneros a national network of grassroots donors and ample earned media, but Cuellar and his supporters maintained that it would not translate into local support in a district that is heavily rural and dependent on oil.

Her near-miss is likely to embolden a score of liberal primary challengers hoping to take out House Democrats, including two later this month. In Illinois, Democrat Marie Newman is making another run at Rep. Dan Lipinski who, like Cuellar, also opposes abortion rights. And in Ohio, Rep. Joyce Beatty faces a stiff challenge from consumer advocate Morgan Harper.

Cuellar, perhaps spooked by the rash of Democratic incumbents who fell last cycle, assembled a formidable campaign apparatus. Former staffers of Joe Crowley, who lost to Ocasio-Cortez, reached out last year to Cuellar's team to offer advice.

While Cisneros raised over $1 million — and had sizable help from an EMILY’s List affiliate — Cuellar still dwarfed her in spending. Buoyed by a massive campaign war chest bolstered by years of uncompetitive races, he dropped $2.3 million as of mid-February and still had $2 million left in the bank for the final stretch.

Cuellar’s allies said he took the race seriously from the start, deploying television, radio, digital and newspaper ads. The campaign hired paid canvassers in December and said it has knocked on 100,000 doors.

For much of the race, Cuellar scoffed at the idea that Cisneros could gain traction. The campaign polled several times, according to Cuellar strategist Colin Strother, and maintained a strong lead throughout.

Cuellar refused to debate Cisneros during the race but he did blast out negative mailers and one contrast television ad, knocking her as a New York transplant. One mail piece showed a picture of Cisneros in front of the Brooklyn Bridge. (Cisneros was born in Laredo and attended law school in Austin before completing a legal fellowship in Brooklyn.)

His campaign suggested that at least some of his spending was to protect his South Texas brand. The Cuellar family is an institution in Laredo, which makes up a bulk of the electorate. His brother is the Webb County sheriff, and his sister is the county’s tax assessor-collector.

Part of Cisneros’ strategy was to significantly change the electorate, bringing in young and low-propensity voters. She campaigned on college campuses throughout the district.

Establishment forces banded together behind Cuellar. House Speaker Nancy Pelosi both endorsed him and campaigned with him in the district. And he no doubt benefited from a policy change at the Democratic Congressional Campaign Committee that essentially blacklists campaign consultants who work to oust a sitting member of the conference.

His campaign also received support from the U.S. Chamber of Commerce, the oil industry and the conservative Koch network — a donor list that Cisneros said was further proof that Cuellar is out of touch with such a Democratic-leaning district.

Cuellar was one of three members who faced credible primary challengers on Tuesday.

In California's Central Valley, Rep. Jim Costa, a fellow Blue Dog Democrat, appeared on track to stave off Fresno City Council Member Esmeralda Soria. He held a 20-point lead over Soria with 100 percent of precincts reporting, though the AP had yet to call the race.

And in North Texas, GOP Rep. Kay Granger easily dispatched primary challenger Chris Putnam, delivering a boon to the GOP establishment as well as the forces within her party working to combat the dwindling ranks of Republican women.

Granger led Putnam, a technology executive backed by the conservative Club for Growth, by 16 points, with 100 percent of precincts reporting.

The comeback bids

Three former GOP members attempted comebacks Tuesday night. One has advanced to a primary runoff in May, while the fate of the other two was still uncertain as of Wednesday afternoon.

Former Rep. Pete Sessions (R-Texas) spurned the suburban Dallas seat he lost last cycle to Democrat Colin Allred to run in a Waco-based district 100 miles south that is more GOP-friendly. He notched a spot Tuesday in the May 26 primary runoff, but the AP had not determined his opponent as of Wednesday afternoon.

In a vacant southern California seat, former GOP Rep. Darrell Issa was locked in an intraparty fight for a spot in the general election to replace former Rep. Duncan Hunter (R-Calif.). Issa retired last cycle rather than run for reelection in a neighboring district, which Democrats ultimately flipped by a double-digit margin.

After a bruising race, Issa and fellow Republican Carl DeMaio were still fighting for the second spot in California's protracted vote count. Democrat Ammar Campa-Najjar was the top vote-getter, but he will be at a disadvantage in November when the Republican vote is not splintered.

Meanwhile, former GOP Rep. Steve Knight was struggling to advance in the special election for the northern Los Angeles seat he lost last cycle to Democrat Katie Hill, who resigned late last year. Knight currently trails Navy veteran Mike Garcia, though the AP has not called the race. Democratic state Assemblywoman Christy Smith took first place but finished shy of a majority, prompting a May 12 runoff.

National Democrats spent upward of $500,000 in an attempt to nudge Knight into the general election, a sign that they believe Garcia would be a more formidable Republican foe.

The Texas battleground

Pierce Bush, the latest member of the Bush clan to run for office in Texas, fell short in an open, suburban Houston battleground district on Tuesday. Bush, a nonprofit leader and grandson of George H.W. Bush, failed to qualify for a GOP runoff, finishing behind Fort Bend County Sheriff Troy Nehls and GOP megadonor Kathaleen Wall.

Bush tried to stake out a compassionate-conservative message in a massive 15-candidate field, but he got a late start and struggled somewhat with his family's messy history with President Donald Trump.

The winner of the May 26 runoff will face Democrat Sri Preston Kulkarni, the 2018 nominee for the seat, in the general election to replace retiring Rep. Pete Olson (R-Texas).

House Democrats and Republicans tapped nominees in some districts targeted by both parties in November. Army veteran Wesley Hunt and businesswoman Genevieve Collins will take on freshmen Democratic Reps. Lizzie Fletcher and Allred, respectively.

Wendy Davis, the 2014 nominee for governor, will face GOP Rep. Chip Roy in a central Texas district.

And in a safe red seat in the Texas panhandle, Ronny Jackson, Trump's former physician, advanced to a Republican runoff with Josh Winegarner.

Laura Barrón-López contributed to this report.