For Jessica Cisneros, not every Democrat is the same. And she's trying to prove that by challenging her former boss.

Rep. Henry Cuellar, a well-connected Democratic incumbent, is now facing an insurgent primary challenge from Cisneros, a 26-year-old immigration attorney who once interned in his Washington office. The centrist congressman has represented Texas' 28th Congressional District, which stretches from Laredo and other border communities along the Rio Grande to the outskirts of San Antonio, since 2005.

"He might have that 'D' next to his name, but in reality he's been walking and talking like a Republican," she told CBS News, referring to Cuellar. "He's Trump's favorite Democrat."

Jessica Cisneros, 26, hopes to topple an eight-term Democrat in a Texas border district. Jessica Cisneros campaign

During his eight terms in the House, Cuellar, a member of the Blue Dog Coalition, has billed himself as a moderate Democrat willing to work with Republicans, especially on immigration issues that affect his border district. He holds anti-abortion rights views, was one of only three House Democrats in 2018 to be supported by the National Rifle Association (NRA) and has supported Republican-led immigration bills, including one that would crack down on so-called sanctuary cities.

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Cuellar, armed with a significant war chest, has managed to defeat all primary challenges in his reliably blue district, where nearly 70% of eligible voters are Latino. By casting him as beholden to big corporations and too cozy with President Trump and other Republicans, however, Cisneros hopes to stage the most formidable primary challenge Cuellar has ever faced.

Her bid has already garnered the support of Justice Democrats, the progressive group that helped New York Rep. Alexandria Ocasio-Cortez pull of her shocking primary win against one of the top Democrats in the House, former Rep. Joe Crowley, in 2018.

Cisneros is one of two candidates the organization has backed to topple incumbent Democrats in blue districts going into the 2020 elections. The group has thrown its support to Jamaal Bowman, a middle school principal from the Bronx, who is looking to strip the Democratic nomination to represent New York's 16th congressional district from 16-term Rep. Eliot Engel, the powerful chair of the House Foreign Affairs Committee.

For Alexandra Rojas, executive director of Justice Democrats, the group's endorsement of Cisneros, who was relatively unknown before her announcement, is part of an effort to overhaul the Democratic Party and bolster its progressive wing.

"Jessica has a deep commitment to the working families of South Texas and has what it takes to lead her district," Rojas said. "Our grassroots progressive movement has an opportunity to build a more accountable Democratic Party by unseating one of the worst amid our ranks and ushering in a new generation of leaders."

Cisneros, the daughter of Mexican immigrants and a graduate of University of Texas at Austin Law School, is hoping to draw sharp contrasts with Cuellar on immigration. She said the her opponent is siding with Republicans to misrepresent the situation at the U.S.-Mexico border, where the border officials have been dealing with a months-long unprecedented surge of Central American families and unaccompanied children seeking asylum.

"We have people like Cuellar who are perpetuating this idea that there is a crisis here on the border and, just like every Republican, just feeding into this notion that everything is falling apart down here," she added.

Her platform on immigration includes ensuring a pathway to citizenship for undocumented immigrants and so-called DREAMers, making the backlog-ridden immigration court system independent from the executive branch and restoring foreign aid to curb poverty and violence in Central America — which Mr. Trump has ended.

Cisneros also vigorously opposes the Trump administration's policy known as "Remain in Mexico" that requires asylum seekers to wait in Mexico while their cases are processed in U.S. courts. The administration has vowed to expand the policy — which is in place at three ports of entry — along the entire southern border, including at the Laredo port of entry in Cisnero's district.

Colin Strother, a spokesperson for Cuellar's congressional campaign, suggested the support around Cisneros' campaign is coming from outside of Texas.

"Rep. Cuellar represents a diverse, socially conservative district and he votes their values — not the party line. Our opponent backs a radical job-killing agenda that would send South Texas back to the Stone Age," Strother added. "We look forward to comparing our values to those of Ms. Cisneros and her New York backers. This isn't the Bronx, this is the border."