On Sunday, Mayor Wu surprised her former aide, Enrique Pepen, after they had both marched in the Dominican Day Parade: She told him she was endorsing him in the four-way District 5 City Council race.

Candidates for public office often happily greet the backing of the mayor of Boston. While mayors, from Tom Menino to Marty Walsh and Wu, have in recent history racked up an uneven win-loss record when it comes to whom they support, the imprimatur does come with upsides. At the very least, a flood of articles trumpets the news and raises awareness as the city approaches a likely low-turnout election with just the City Council on the ballot. The endorsement was first reported by Politico Massachusetts.

In the case of Wu wading into District 5, her endorsement also includes a vote. The mayor, like Pepen, lives in Roslindale, which along with Hyde Park and part of Mattapan make up the neighborhoods included within the district’s boundaries.

Her decision also represents a public break with the incumbent councillor, Ricardo Arroyo, whom she backed in the bitter race for Suffolk County District Attorney, won by Kevin Hayden, a prosecutor appointed by former Gov. Charlie Baker. Arroyo supported Wu in the 2021 mayoral race, after his first pick, acting mayor Kim Janey, didn't make it past the preliminary, and as the chair of the City Council’s government operations committee, he has moved along some of her top priorities, including rent control legislation.

But the last year has brought headlines of a $3,000 ethics violation for representing his brother, former Councillor At-Large Felix Arroyo, in a lawsuit involving City Hall and sexual harassment allegations. In addition, his name was plastered all over two federal investigatory reports into the behavior in office of former US Attorney Rachael Rollins, who was in regular contact with Arroyo and faced accusations that she meddled in the DA’s race.

Along with her support for Pepen, Wu has headlined fundraisers and voiced support for Councillor At-Large Ruthzee Louijeune; former administration aide Henry Santana, also running for at-large; District 1 Councillor Gabriela Coletta (East Boston); District 8 Councillor Sharon Durkan (Beacon Hill, Back Bay and Mission Hill), a longtime Wu campaign adviser; and District 9 Councillor Liz Breadon (Allston-Brighton).

After the Dominican Day Parade, and before Wu attended an annual event in Back Bay honoring gardeners who have landscaped the city’s neighborhoods, she and Pepen headed to a Roslindale park, which provided a leafy backdrop when they posed together for a picture.

In a statement sent out the following morning, Wu called Pepen, who served as the mayor’s executive director of the Office of Neighborhood Services, “exactly the kind of leader we need in government.” She added that Pepen is “thoughtful and kind, creative and tenacious.”

The mayor’s backing came days after Progressive West Roxbury/Roslindale, a local political group, endorsed Arroyo and others on the Council for reelection “by extremely wide margins.” Arroyo lists state Rep. Russell Holmes, a Mattapan Democrat, as his one other endorser in this election cycle.

Arroyo said in a statement responding to Wu’s rebuff that he is “the only candidate in this race with a proven independent and progressive record.” Reached by phone, he declined to comment beyond the statement.

The others in the District 5 race are Jose Ruiz, a retired Boston Police officer who served under Mayor Walsh and lives in Readville, and Mattapan’s Jean-Claude Sanon, who has previously run for the seat. The preliminary, which will narrow the field to two candidates, is set for Sept. 12.

As-yet unnamed people who oppose Arroyo have formed a super PAC, an outside group that can fundraise and spend money in order to support or oppose a candidate as long as it doesn’t coordinate with the candidate. Carrying the name “Enough is Enough,” the super PAC’s paperwork filed with state regulators makes clear its mission: Anyone but Arroyo this fall.

On Tuesday, Pepen told the Reporter that he had been knocking on “thousands and thousands” of doors in the district. “Because of that we’ve been able to really gather our 1s and 2s and identify them in all three neighborhoods of the district,” he said, using campaign parlance for ardent supporters. Pepen has also been endorsed by several labor unions, including UNITE HERE Local 26.

Asked if voters bring up the embattled incumbent, Pepen said the more common theme he hears are complaints about the divided and pugnacious 13-member body and a desire for fresh voices. “They’re just unhappy with the City Council,” he said.

Ruiz, who has the support of IBEW Local 222, the Dorchester-based firefighters union Local 718, and the MBTA carmen’s union, also criticized the infighting in City Hall. “In the last two years, we have seen divisions bring the work of the Boston City Council to a grinding halt, with factions forming and one-sided pitted against the other,” he wrote in response to a Reporter questionnaire sent to all candidates. If elected, he’ll work to “bring the council together,” he said.

Arroyo, asked earlier this month by the Reporter whether voters he speaks to bring up the headlines, said, “It doesn’t not come up, but it’s very minimal.” Voters he talks to bring up topics like speeding in the streets and public schools, he said. “What has been prevalent are conversations about global warming and climate and how that’s impacting our city and the country.”

In the questionnaire, the District 5 candidates were also asked to grade their high-profile constituent. All the candidates, with the exception of Sanon, who didn’t respond, gave Mayor Wu a passing grade.

Ruiz added that he believes “we still have much work to do. In particular, dealing with the housing crisis, investing more to keep our communities safe, making actual and lasting progress, improving our education system, and more.”

Pepen said his former boss is “taking chances and trying out exciting new things we haven’t tried before…There’s always room for improvement, and no elected official is perfect. As time goes by, I believe Mayor Wu will continue to improve.”

Arroyo, who submitted his responses before news of Wu’s Pepen endorsement, said he agrees more with what the mayor has done than not. “She has led on rent stabilization, much needed environmental reforms, and is leading on reforming the [Boston Planning and Development Agency], all things I support,” he wrote. “We have, however, disagreed on the need for an elected school committee and reallocating funding from the Boston Police Department to social services and programs that help reduce the impacts of poverty, which is the root cause of most crime in our city.”

Over the weekend, Wu added another to the list of their disagreements: Who she believes is the best candidate for District 5.

This story has been updated to correct the timing of Mayor Wu informing Pepen of her endorsement.