* Correction appended.

WASHINGTON — Not many people understand South Texas.

It's one of the handful of blue pockets in the state, but unlike the others, it's not clustered in an urban center. The congressional districts that represent it encompass small border cities and ranch lands alike. Like other heavily Hispanic areas, the number of young voters grows each election, and what that means for the Democratic Party is uncertain.

But a spirited primary campaign in the district long held by U.S. Rep. Henry Cuellar, a moderate Democrat from Laredo, could test if and how the politics of the area are changing. Cuellar, who has served in Congress since 2004, isn't too different from the other Democrats who represent the Rio Grande Valley in Washington and Austin. But now he’s facing a challenge from a former intern who is running on a progressive platform.

Jessica Cisneros, a 26-year-old immigration attorney, has pushed for proposals like "Medicare for All," gun control and the Green New Deal. Cuellar has argued that those ideas don’t align with the beliefs of the district and that the bulk of Cisneros’ support comes from people outside the region. The race has become one of the most closely watched in the country early in the primary season.

Texas’ 28th Congressional District spans from western Hidalgo County, right outside the McAllen city limits, and up north to Laredo and the eastern San Antonio suburbs. In 2016, it went to Hillary Clinton by 20 percentage points, and former U.S. Rep. Beto O'Rourke won it by similar margins in the 2018 Senate election.

Cisneros said that’s evidence that dispels the myth that South Texas is conservative, and even more reason a right-leaning Democrat, or “Trump’s favorite Democrat,” as Cisneros calls Cuellar, should be replaced with a more progressive representative.

“I haven’t had a single person disagree with me in terms of my policies,” Cisneros said. “It’s not surprising to me to see folks super shocked that somebody who is running for office is at their doorsteps. There are people who simply respect that I’m putting the effort.”

Cuellar declined to comment for this story.

Both candidates have pointed to the other’s financial support as a sign that their opponents are out of touch with the district.

Cuellar has received money from the National Rifle Association, a political action committee associated with Koch Industries and the GEO Group, the private prison company that funds migrant shelters where several migrants have died. But he also takes in a large sum of money from donors within the district — and has tried to portray Cisneros’ support as largely driven by outsiders.

Few of Cisneros’ reportable donations come locally, which the Cuellar campaign has been quick to point out.

Cisneros, a recent law school graduate, is also not licensed to practice law in Texas because she took the bar exam in New York. (Texas will, however, soon switch to the Uniform Bar Exam, which will make her score transferable.)

The district is about 80% Latino and consistently votes Democrat. And although Cuellar has consistently won elections by healthy margins, he has never had a primary challenger in a district where elections are often decided in the primary.

And Cisneros has been effective in making her presence known.

She has the backing of several influential progressive organizations: EMILY’s List, a group that backs female Democratic candidates who support abortion rights; the Sunrise Movement; and several unions.

She has also picked up endorsements from presidential candidate Elizabeth Warren and two of Cuellar’s most outspoken liberal colleagues in the House: U.S. Reps. Alexandria Ocasio-Cortez and Ayanna Pressley. Neither House endorsement was particularly unexpected, though, considering both Ocasio-Cortez and Pressley also received support from Justice Democrats, a progressive organization that's also backing Cisneros.

As her campaign has gained attention, several media outlets have dubbed her “the next AOC,” a title she doesn’t particularly promote herself.

“A lot of folks tend to draw comparisons between us two,” Cisneros said. “We are both young brown women who are underdogs and going up against somebody who has been in office for so long and are trying to do our best to help the Democratic Party and find true representation for our districts. But I think that’s where the similarities end. We’re our own people. Her campaign and the way she ran hers is going to be a lot different than the way we run ours.”

But unlike former U.S. Rep. Joe Crowley, the longtime Democratic incumbent Ocasio-Cortez unseated, Cuellar lives in his district — and he’s there often. His name is known particularly well in Laredo, where Cisneros is also from, and where his brother is the county sheriff.

On Capitol Hill, Cuellar's allies do not detect any sense of panic. Typically when under political threat, members solicit colleagues for campaign donations. It was an effective tactic former U.S. Rep. Gene Green of Houston deployed during his competitive primary in 2016.

Cuellar has not asked some of his closest colleagues for donations. But that could well be because he already has a daunting amount of money saved up in his campaign account. Cuellar had over $3.2 million on hand in October, compared with Cisneros' $317, 000, according to their most recent Federal Election Commission filings.

He undoubtedly has the backing of party leadership. At The Texas Tribune Festival in September, House Speaker Nancy Pelosi voiced strong support for Cuellar, adding that she is “very, very proud of Henry's work in Congress, and I'm proud to support him — even if I didn't have a policy of endorsing incumbents." Cheri Bustos, the head of the House Democrats' campaign arm, said Cuellar “is going to be fine,” rejecting the notion that he is at all vulnerable.

Besides being overwhelmingly Latino, the district also encompasses some of the state’s poorest ZIP codes. That’s not unrelated to the fact that it also has some of the lowest voter turnout rates in Texas.

Although that would appear as an obstacle for a new candidate, Cisneros sees it as a new frontier.

“I definitely don’t see it as a problem; I see it as an opportunity,” she said.

Although Cuellar’s war chest outweighs the amount Cisneros has raised so far, she still outraised his campaign by $100,000 in small-dollar donations. Donations under $250 aren’t trackable, so it’s not clear how many of those are from voters in the district, but that distinction could make a huge difference.

“It is a poor district. … These people live humbly,” said Moses Mercado, a Democratic lobbyist who is close to a number of Texas members of Congress. “It’s not a rich district that you could raise money from. The fact that she raised more than him is significant.”

The election is also taking place during the presidential primary, which tends to turnout more voters than usual. As a member of the House Appropriations Committee, Cuellar has voted several times to approve border wall funding, which is a point of contention for many of his constituents, particularly in the Rio Grande Valley. The expected backlash toward the Trump administration could steer voters away from a candidate who leans conservative for a Democrat, like Cuellar, whose votes align with Trump about 70% of the time.

“The new voters that they bring out are not going to be Cuellar voters,” Mercado said. “This is going to be a high-turnout election, and that’s going to matter. It’s going to catch people by surprise. I think we have a base that’s very motivated, very angry.

“This is not the year to be complacent.”

Abby Livingston contributed to this story.

Correction: This story has been updated to reflect that U.S. Rep. Henry Cuellar has received money from a political action committee associated with Koch Industries.