Mayor Lori Lightfoot on Saturday endorsed Marie Newman in her bid to unseat U.S. Rep. Dan Lipinski, D-Ill., giving Newman’s campaign a key progressive boost ahead of the Chicago area’s biggest Democratic primary contest next month.

Lightfoot previously signaled her support for the challenger last month, declaring it was Lipinski’s “time to leave” after the eight-term, anti-abortion incumbent co-signed a legal brief asking the U.S. Supreme Court to revisit the Roe V. Wade decision.

But after declining to name her candidate, Chicago’s first black female and openly gay mayor formally announced she was “proud to endorse Marie Newman” in the heated 3rd Congressional District primary.

“I know that when it comes to protecting hardworking immigrants and refugees who have helped build and grow our city and region, Marie Newman will stand in solidarity with these important communities,” Lightfoot said in a statement.

“It’s time for the people of the 3rd District to have a real Democrat representing their values,” Lightfoot said, adding that “it’s no wonder some refer to Lipinski as ‘Trump’s favorite Democrat.’

“Lipinski voted against Obamacare, mocks efforts to combat climate change, and is actually still opposed to marriage equality,” Lightfoot said. “That’s right, if it were up to Dan Lipinski, I wouldn’t be able to marry my wife.”

In response, Lipinski’s campaign said in a statement that the mayor “is certainly entitled to her opinion, but hers is not the sentiment of the mayors and managers who actually live and work in the Third Congressional District and are supporting Congressman Lipinski.”

Lightfoot announced her endorsement at the Beverly Unitarian Church in the city’s 19th Ward, which Newman’s campaign said provided their best turnout of any city ward in her first bid to take Lipinski’s seat in 2018. It’s also where Lightfoot won nearly 85% of the vote over Cook County Board President Toni Preckwinkle in last year’s mayoral runoff election.

But it remains to be seen how the big-name endorsement will play out for Newman in the rest of the 3rd District, which, besides the city’s Bridgeport neighborhood, mostly spans the southwest suburbs — and is home to many conservative Democratic voters.

“Congressman Lipinski has been endorsed by 28 mayors from across all areas of the Third District who are supporting him because of his demonstrated ability to resolve local issues and deliver federal resources,” his campaign said. “The Congressman’s priorities reflect the issues most important to the residents of his district, which are lower healthcare and prescription drug costs, improved education, environmental protections, and job growth.”

The March 17 primary represents a grudge match from Newman’s 2018 challenge, which she lost by just 2,145 votes — or 3.5% of the primary vote — in the toughest political battle Lipinski had ever faced. Newman refused to concede on Election Night, and the bad blood between the two only has only continued to simmer since then.

Lightfoot has already recorded a digital ad for the Newman campaign and will be “very active” supporting the challenger in the weeks ahead, Lightfoot political director Dave Mellet said.

It’s the highest profile endorsement yet for the freshman mayor. Lightfoot has joined other leading Illinois Democrats in supporting Cook County State’s Attorney Kim Foxx’s re-election bid, but she has not yet publicly chosen a horse in her party’s presidential sweepstakes. Lightfoot has met or spoken with a number of the White House hopefuls and dismissed those who have not bothered to reach out to her.

The Newman backing helps Lightfoot burnish her progressive credentials, but it also puts her at odds with state House Speaker Mike Madigan, a strong Lipinski backer whose support Ligthfoot will need in her efforts to secure a Chicago casino.