New York City Public Advocate Jumaane Williams is endorsing Manhattan Assemblymember Yuh-Line Niou, a left-leaning candidate in the competitive and crowded contest for a rare open House seat representing one of New York’s most liberal swaths.

Williams confirmed the news to Gothamist on Sunday, and Niou’s campaign plans to announce his endorsement Monday morning. Williams — a prominent progressive city lawmaker who recently came in second in the Democratic gubernatorial primary — is the first citywide elected official to weigh in on the race.

Niou, who endorsed Williams for governor, is one of 12 candidates vying for the newly drawn 10th congressional district, covering Lower Manhattan and northwest portions of Brooklyn. Former Mayor Bill de Blasio dropped out last month, citing a lack of support.

Williams’ endorsement could help further cement Niou’s standing as the most progressive candidate in the race. It comes five days before early voting begins on Saturday, and two days before the first televised debate, which is being co-hosted by WNYC and NY1.

Williams, who previously served as a Brooklyn City Councilmember before becoming public advocate, told Gothamist he and Niou shared a “synergy” on a variety of issues, including affordable housing, hate crimes, and ways to tackle gun violence without increased policing.

He said he was also especially drawn by the candidate’s willingness to challenge the Democratic establishment and cited her early criticism of former Gov. Andrew Cuomo, who resigned nearly one year in disgrace following sexual harassment complaints.

“Silence oftentimes can be complicity,” Williams said. “You didn’t find that with Yuh-Line, and for a sitting elected state official to be able to do that shows a lot of courage, in my opinion.”

Similarly, Niou praised Williams as a fellow progressive who she deeply admired and with which she shared “a political home.”

“I think that he's been an incredible voice and an incredible public advocate,” she said.

Recent polls have suggested that the race is narrowing around Niou, Carlina Rivera, a Manhattan City Councilmember, and Daniel Goldman, an MSNBC legal analyst and former prosecutor who led the first impeachment hearing against former President Donald Trump. Although the three candidates largely agree on major issues, Niou, an Asian American, and Rivera, a Latina, have emphasized the importance of amplifying the voices of underrepresented communities in the district.

Williams added that Niou has not raised any significant money from the real estate industry, something of a litmus test for progressives in recent years.

The endorsement for Niou adds to a string of support from the progressive flank of the Democratic Party. Last month, she received the support of the Working Families Party (WFP), an influential progressive organization that also backed Williams in his race for governor. The endorsement was seen as providing a fundraising boost for Niou, who’s so far raised over $241,000, the least of the top four candidates, according to campaign filings.

The WFP has historically held sway in portions of the new district. According to an analysis by John Mollenkopf, director of the Center for Urban Research at CUNY, voters in the district cast the highest share of votes for Biden on the WFP line — 13.5% — compared to other congressional districts in New York. Citywide, the proportion of voters voting on the WFP line was 7.2%.

Given the crowded and up-for-grabs nature of the race, many experts have said the outcome could hinge on endorsements.

Rivera has also captured several key endorsements, including that of Rep. Nydia Velazquez — a veteran lawmaker whose existing Brooklyn district overlaps with the 10th congressional district — Brooklyn Borough President Antonio Reynoso and several of her colleagues in the City Council.

And on Sunday, Rivera announced she had won the endorsement of Manhattan Borough President Mark Levine.

Her news followed that of former Rep. Liz Holtzman, another candidate in the race, who earned the endorsement of the Daily News.

In what may be the most anticipated endorsement, the New York Times editorial board has yet to publicly back a candidate in the race. Although the Times has a mixed record on choosing winning candidates, it may have outsized influence in the 10th congressional district and 12th congressional district, which covers the Upper East and West sides, because of the number of affluent liberal residents.

Aside from endorsements, experts say traditional campaigning in the form of door knocking and phone banking will also be critical in a race expected to draw low turnout.

Niou told Gothamist that she has 1,000 volunteers working for her campaign.

Acknowledging that she did not have the same resources to run expensive TV advertising as some of her competitors, she said she believed the ground game would make the difference in the race.

“I’m betting on people,” she said.