Sen. Bernie Sanders (I-Vt.), a candidate for the Democratic presidential nomination, has yet to endorse Texas Democrat Jessica Cisneros, an immigration attorney challenging Rep. Henry Cuellar, one of the House’s most conservative Democrats, in Texas’ 28th Congressional District.

Sanders’ silence in what has become one of the most closely watched primary challenges in the country has puzzled and frustrated some of the progressive activists who are typically his allies.

The Vermont senator’s absence is particularly glaring because of the breadth of support Cisneros already enjoys. The Laredo native, who at 26 would be the youngest-ever woman elected to Congress, is backed by Sen. Elizabeth Warren (D-Mass.), Reps. Alexandria Ocasio-Cortez (D-N.Y.) and Ayanna Pressley (D-Mass.), and a broad array of progressive organizations and labor unions that typically stick with incumbents. The latter group of endorsers includes EMILY’s List, the Planned Parenthood Action Fund, a branch of the Communication Workers of America, MoveOn Action, and the League of Conservation Voters.

Win McNamee/Getty Images Sen. Bernie Sanders (I-Vt.) has intervened in a number of competitive Democratic primaries, but he has yet to pick sides in a closely watched House race in Texas.

“It’s going to be really hard to build a political revolution that gets 51 votes in the Senate if you’re not willing to carry a carrot and a stick and one of those sticks is primary challenges against conservative Democrats,” said Sean McElwee, co-founder of the progressive think tank, Data for Progress, which has conducted polling for Warren. “It’s going to be incredibly hard to scare [Sens.] Tom Carper and Chris Coons if you’re not willing to take on Henry Cuellar.”

“I’m genuinely baffled,” added McElwee, who has not formally endorsed in the Democratic presidential primary. “Why is Warren supporting the Henry Cuellar challenger and Bernie isn’t?”

Having the endorsement of someone like Bernie Sanders would be incredibly useful to her. Yasmine Taeb, Virginia DNC member

A progressive Texas activist and Sanders volunteer, who declined to be named for professional reasons, echoed McElwee’s arguments. The activist noted that while Sanders had endorsed progressive candidates in several contentious state and local primaries across the country, “none of those can pass his agenda at the federal level.”

“Bernie needs reinforcements in Congress and this one is a no-brainer,” the activist concluded.

McElwee worries that it reflects Sanders’ “institutionalist” streak, noting that the Vermont senator has also refused to embrace the elimination of the filibuster. (Warren and South Bend, Indiana, Mayor Pete Buttigieg have both come out in support of ending the practice.)

More urgently, McElwee fears a repeat of the Virginia legislative primaries in June, when human rights attorney and Democratic National Committeewoman Yasmine Taeb fell short in her bid to unseat the pro-corporate, state Senate Democratic Leader Dick Saslaw by a few hundred votes. Sanders did not endorse Taeb, a supporter of his in the 2016 election, though Our Revolution, a liberal advocacy group that emerged from his first presidential bid did.

Taeb, who said she is leaning toward endorsing Sanders again, believes she would have benefited from his help and encouraged him to get involved in Cisneros’ race.

“Having the endorsement of someone like Bernie Sanders would be incredibly useful to her,” Taeb said.

It would be great to see @BernieSanders put his political revolution into action by endorsing progressive populist primary challengers around the country, like @JCisnerosTX or @Booker4KY.https://t.co/OZwzaVeZd3 — Evan Weber 🌅🔥 (@evanlweber) December 11, 2019

In a statement, Cisneros likewise said she would welcome Sanders’ help.

“Our campaign is focused on our own race down here in South Texas,” Cisneros said in a statement. “Of course, we’d welcome an endorsement from any of the leading presidential candidates who share our values, including Senator Sanders.”

Cisneros, the daughter of Mexican immigrants, is running on a progressive platform that Sanders made mainstream, including enacting Medicare for All, passing a Green New Deal to fight climate change, and making public college tuition-free.

In fact, it’s possible that Cisneros would not be running at all were it not for Sanders’ 2016 bid. Justice Democrats, a left-wing group formed by alumni of that Sanders campaign that is targeting moderate and conservative Democrats in safe blue seats, recruited her to run. She returned to Texas’ 28th from New York City, where she was practicing immigration law, and announced her candidacy in June.

Veronica Cardenas / Reuters Democrat Jessica Cisneros campaigns in Laredo, Texas, in October.

Cuellar, 64, who has represented the heavily gerrymandered South Texas district since 2005, is indeed one of the most conservative Democrats in the House. The NRA gave him an A rating in 2018 for his record of opposition to tougher gun safety regulations. NARAL Pro-Choice America, by contrast, granted him a score of 15% in 2018 for votes to restrict abortion rights, including for some “personhood” bills that effectively created legal rights for unborn fetuses.

Cisneros has built her campaign around the notion that the district, where Donald Trump lost to Hillary Clinton in 2016 by nearly 20 percentage points, deserves a stronger fighter for its interests. She is fond of calling Cuellar “Donald Trump’s favorite Democrat” and has even sought to blast Cuellar for his friendship with Energy Secretary Rick Perry, who is a figure of interest in the impeachment inquiry.

But Cuellar, who is backed by House Speaker Nancy Pelosi (D-Calif.), maintains that his moderate style is a better fit for the low-income, predominantly Latino district, which stretches from the San Antonio area down to border cities like Laredo, Mission, and Rio Grande City.

In a C-SPAN interview on Friday, Cuellar mocked Cisneros as an out-of-touch radical. He called the left-wing group backing her “Justice Socialists,” and claimed that the “Green Deal” would kill thousands of jobs in the district.

Tom Brenner/Getty Images Rep. Henry Cuellar (D-Texas) speaks in favor of the U.S.-Mexico-Canada trade agreement at a rally in Washington in September.

Notwithstanding Cuellar’s efforts to paint Cisneros as radical, Waleed Shahid, a spokesman for Justice Democrats, said that Sanders’ endorsement would be helpful to her. “Cuellar and his Republican and corporate donors will be a bulwark against any progressive legislation so of course we hope Bernie and other leaders in the party and progressive movement would endorse her,” he said.

But time is running out for Sanders. Texas Democrats vote in primaries to decide their House nominees ― as well as the party’s presidential nominee ― on Super Tuesday, March 3.

It is unclear why Sanders, who has used his platform and resources to get involved in many other contentious primary races, including by backing Marie Newman’s challenge against conservative Democratic Rep. Dan Lipinski of Illinois, is staying on the sidelines. His campaign declined a request for comment.

In an interview with The Intercept’s Ryan Grim in October, Sanders wholeheartedly endorsed the project of progressive primary challenges against more conservative incumbent Democrats, claiming “we need to elect a House and a Senate which is as progressive as it possibly can be.” But when Grim pressed him on specific races, such as Alex Morse’s effort to unseat Rep. Richard Neal of Massachusetts, Sanders declined to engage. “I’m not going to look at it race by race,” he said.

It is possible that Warren’s endorsement of Cisneros, followed by a joint campaign rally, has given the campaign pause. Politicians sometimes fear that endorsing a candidate who is unlikely to reciprocate by endorsing them back can make them seem weak.

But McElwee argues that the potential benefit of a win for Cisneros with Sanders’ blessing far outweighs the potential risk. Her victory would undermine the idea that there is no constituency for progressive policies in rural pockets of the country ― and strengthen the case that Sanders is capable of breaking through in unexpected places, according to McElwee.

“Taking on a corporate Democrat in March would prove a really strong signal for progressives, both in House primaries but also in the presidential primary,” he said.