WASHINGTON — Just weeks before voting starts in what is expected to be U.S. Rep. Henry Cuellar’s toughest re-election fight in years, a dark money group is flooding his South Texas district with ads supporting him.

The nonprofit American Workers for Progress has poured more than $720,000 into ads in Austin, San Antonio, Laredo, McAllen, Brownsville and Harlingen, according to Advertising Analytics, a private firm that tracks political advertising.

Little is known about the newly formed group. A longtime Democratic operative listed as its president declined to comment. Cuellar's campaign spokesman says he knows nothing about the group, as does the state Democratic party.

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Nonetheless, the heavy spending is a sign of how nervous some in Cuellar's orbit are that Jessica Cisneros, a 26-year-old immigration attorney from Laredo who has become a star in progressive circles, could end his 15-year stint in Congress.

The race will provide an early indication in 2020 of the electability of several progressive hopefuls in Texas. They argue that a key to turning the state blue will be reaching voters who have not made a habit of voting in the past, especially young and minority voters, who they believe can be energized by an unflinching progressive message.

Cisneros has raised nearly $1 million in her insurgent effort to take down Cuellar — an impressive haul for a political newcomer in a district long considered safe — and she’s drawn major endorsements from progressive groups, including Justice Democrats, the group that helped U.S. Rep. Alexandria Ocasio-Cortez defeat a longtime incumbent in New York.

Cuellar already has more than $3 million on hand to defend himself, as well as the backing of the Democratic establishment, including House Speaker Nancy Pelosi. The dark money spending was first reported by POLITICO.

“That’s real money, as they say,” said Ben Ray, a spokesman for EMILY’s List, another progressive organization that is backing Cisneros. “He’s got three million damn dollars on hand and his team says they’re cruising, I didn’t think he needed help.”

For subscribers: Texas Rep. Henry Cuellar moves to stave off primary challenge from progressive Cisneros

Cisneros’ campaign was more pointed about the dark money in the race.

"Congressman Cuellar has spent his career protecting the interests of his corporate donors and Republican special interests instead of the people of South Texas,” said campaign manager Danny Diaz. “We know he's trying to hide the groups that have stood by him his entire career — so it's no surprise that no one is being transparent about where these hundreds of thousands of dollars are coming from."

American Workers for Progress is led by Gilberto Ocañas, a former deputy executive director of the Democratic National Committee and longtime Democratic operative. Ocañas declined to comment.

“I've never heard of them and don't know anything about them,” said Colin Strother, Cuellar’s campaign spokesman. “I do know that our opponent's entire campaign has been orchestrated, directed, staffed, and funded by an out-of-state special interest group.”

American Workers for Progress so far has spent more than $340,000 in San Antonio, more than $104,000 in Laredo, more than $242,000 across the Harlingen-Weslaco-Brownsville-McAllen markets and more than $27,000 in Austin, according to Advertising Analytics.

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The ads focus on health care: “Congressman Henry Cuellar fought to protect Obamacare and is standing with Nancy Pelosi to lower drug prices,” it says. “Henry Cuellar is delivering, fighting for quality healthcare and lower drug prices Texas families can afford.”

ben.wermund@chron.com