Rep. Alexandria Ocasio-Cortez (D-N.Y.) joined Sen. Elizabeth Warren (D-Mass.) on Tuesday in endorsing Democrat Marie Newman, who is vying to unseat Rep. Daniel Lipinski (D-Ill.) in a primary next year.

Ocasio-Cortez’s support for Newman marks her first official backing of a primary challenger over an incumbent Democrat, a move that puts her at odds with the House Democrats’ campaign arm, which always supports the sitting lawmaker in a primary contest.

When Ocasio-Cortez came to Congress this year, after ousting longtime incumbent congressman Joseph Crowley in a primary of her own, she vowed to back liberal candidates over Democrats who stake more conservative positions.

Newman, like Ocasio-Cortez in 2018, is backed by the Justice Democrats, a group founded by alumni of Sen. Bernie Sanders’s 2016 presidential campaign. They seek to mirror his insurgent run against Hillary Clinton with congressional primary candidates.

“I am thrilled to receive the endorsement of Congresswoman Alexandria Ocasio-Cortez,” Newman said in a statement released by the Justice Democrats. “Her unwavering dedication to fighting for social, climate and economic justice is a true inspiration. I look forward to working with Congresswoman Ocasio-Cortez and other Progressive Caucus members in Congress to fight for policies that make life safer and more affordable for people in my District.”

Lipinski, who has held his congressional seat since 2004, is one of the last remaining conservative Democrats in Congress who oppose abortion rights. That has alienated him from most of the party and prompted other, more centrist Democrats, such as Sen. Kirsten Gillibrand (D-N.Y.), to support his challenger.

“At a time when our reproductive rights are under attack, Illinois deserves a leader with an unwavering commitment to fighting for reproductive health care. I’m proud to endorse @Marie4Congress,” Warren tweeted last week. “She understands this isn’t a moment to back down — it’s time to fight back.”

Lipinski, responding to Ocasio-Cortez’s support for his opponent, said in a statement that the endorsement “makes crystal clear that Ms. Newman is an extreme candidate who is completely out of step with the voters of Illinois’ Third Distrct who do not want to be represented by a fifth member of the ‘Squad’.”

The “squad” reference is to Ocasio-Cortez and three other Democratic minority freshman congresswomen, Ilhan Omar (Minn.), Ayanna Pressley (Mass.) and Rashida Tlaib (Mich.), who have banded in support of each other.

Ocasio-Cortez called Lipinski’s reaction “aggressive,” and said “statements like these are so casually thrown from corporate candidates towards grassroots ones, yet they never get branded as ‘divisive,’ despite their rhetoric.”

Ocasio-Cortez is likely to throw her considerable star power behind other liberal Democratic primary candidates. Warren has also endorsed Justice Democrats-backed Jessica Cisneros over Rep. Henry Cuellar (Tex.), another antiabortion Democrat.