WASHINGTON — New York Rep. Alexandria Ocasio-Cortez is the latest high-profile Democrat to take the unusual step of opposing a colleague‘s re-election by endorsing a progressive challenger to Illinois Democratic Rep. Dan Lipinski.

Ocasio-Cortez, who herself defeated a long-serving Democrat in a primary, backed insurgent Marie Newman on Tuesday against Lipinksi, a moderate who has broken with party orthodoxy on abortion and other issues while representing a Democratic-leaning district in Chicago.

"I am thrilled to receive the endorsement of Congresswoman Alexandria Ocasio-Cortez,” Newman said in a statement. "Her unwavering dedication to fighting for social, climate and economic justice is a true inspiration."

Illinois' 3rd Congressional District candidate for Congress, Marie Newman, arrives to vote in the Democratic Party's congressional primary election at the Lyons Township in La Grange, Illinois on March 20, 2018. Kamil Krzaczynski / Reuters

Ocasio-Cortez joins some of the country's most prominent progressive names in backing Newman, including presidential candidates Sen. Bernie Sanders, I-Vt., and Sen. Elizabeth Warren, D-Mass., Rep. Ro Khanna, D-Calif., and liberal groups like Planned Parenthood, Emily's List, MoveOn, and Indivisible.

Newman challenged Lipinski last year and fell short, but quickly declared she would run again in 2020.

In the re-match, Newman has garnered institutional support quicker than she did in 2018, but Lipinski is also better prepared for the challenge and vowed to fight hard.

In a statement, Lipinski called Newman "an extreme candidate who is completely out of step with the voters" and said his constituents "do not want to be represented by a fifth member of the 'Squad,'" referring to the group of four progressive women lawmakers that includes Ocasio-Cortez.

"The Democratic Party — and our country — cannot afford an obstructionist 'Tea Party of the Left' when we need to focus on winning this next election and passing policies that will truly help working families and all who are struggling in America today," Lipinski added, slamming Newman's support for "Medicare for All" and the "Green New Deal."

Newman responded to Lipinski on Twitter, saying he's the one who is "extreme and out of step" for voting against the Affordable Care Act and "parroting absolutely false [President Donald] Trump talking points attacking Medicare for All."

Ocasio-Cortez also fired back at Lipinski, tweeting, "Aggressive statements like these are so casually thrown from corporate candidates towards grassroots ones, yet they never get branded as 'divisive,' despite their rhetoric."

The congressman has the backing of party campaign leaders in Washington, who have threatened to blacklist any vendors who do work for Newman's campaign.

High-profile Democratic primaries are becoming more common, thanks in part to the attention garnered by Ocasio-Cortez and others, although the New York lawmaker herself has not been as quick to support primary challenges as some expected she would.

“We are so proud that Marie Newman is the first Justice Democrat of this cycle to receive an endorsement from Congresswoman Alexandria Ocasio-Cortez,” said Alexandra Rojas, the executive director of Justice Democrats, which has supported many of the anti-establishment primary bids. “The momentum is growing in our movement to make the Democratic Party fight for solutions as big as the problems we face and create a party of voters, not corporate donors.”