A few hundred supporters of Dorchester state Rep. Marty Walsh’s mayoral campaign assembled at the intersection of L Street and Broadway on Thursday afternoon, spilling out onto the street, hours after South Boston state Rep. Nick Collins endorsed a rival at the Oliver Perry School. The gathering was a show of force, quickly thrown together as word spread of Collins’s endorsement of City Councillor At-Large John Connolly.

In the small world of Boston politics, Collins’s move has been analyzed and picked apart again and again over the last 48 hours, and most political observers who are close to Walsh and Collins view it as an act of political vengeance, since Walsh did not publicly back Collins’s run for state Senate earlier this year.

Union members -- many of whom had supported Collins in his unsuccessful Senate run and are now backing Walsh, a longtime labor leader -- are furious at the South Boston lawmaker. That apparently includes Walsh’s cousin, who shares his name but has a different initial: “F.” instead of the state representative’s “J.”

As Connolly supporters scattered after a press conference outside the Perry School, a car drove slowly down East 7th Street, past the small of politicians and their staffers getting ready to head to the next event. The car, with a “Marty Walsh” bumper sticker on its back window, went up the street, and then turned around to come back down, stopping a few feet away from City Councillor At-Large Connolly, Collins and Collins’s father Jim, a former Charlestown state representative.

Gripping the steering wheel, Martin F. Walsh, who heads up Laborers Local 223, made a remark that could not be heard by the reporter nearby. Connolly grimaced and told him to stop, and the car drove away.

The councillor would not confirm what was said. State Rep. Walsh deferred comment to his spokesperson, Joyce Linehan, who said the cousin was not speaking for the campaign and called him an “independent player” who had worked hard to elect Collins to the state House and had endorsed him for the state Senate. (Two other Democrats were running in the primary: Linda Dorcena Forry, a Dorchester lawmaker who would go on to win the primary and the general, and South Boston entrepreneur Maureen Dahill.)

But the incident underscores how politics is personal, as well as local. And it showed how a political ally on Wednesday could become a political enemy on Thursday.

The endorsement’s impact on the mayoral race remains unclear. Many people who backed Collins in the state Senate run signed up with Walsh earlier in the mayoral race, including Collins’s State House aide, Jacob Bombard. Other South Boston operatives, like Anthony Gilardi, who once served as Mayor Thomas Menino’s neighborhood liaison, are also on Team Walsh.

At a Florian Hall fundraiser for Walsh on Thursday night, after the stand-out on L Street, a large crowd included former members of the South Boston delegation, including Jack Hart, who left earlier this year for a job at a law firm, and Brian Wallace, who retired in 2010 and whose State House seat Collins now holds.

But when a race has the possibility of coming down to just a few hundred votes or so, every little bit counts. “This is a big day for the campaign,” Connolly said before the incident, surrounded by about two dozen supporters outside the Perry School, including Ed Flynn, the son of former Mayor Ray Flynn. “It really helps us build momentum going down the stretch.”

Collins said he plans to introduce Connolly to his South Boston supporters and constituents. “I’m going to do what I’ve done for other candidates in the past, what I’ve done for Elizabeth Warren,” Collins said. “Introducing John to, even though he’s been a city councillor for six years, to people I know and helping make the case for his candidacy across South Boston. I think he has a strong case to make, particularly his work on the schools.”

Collins said he told the firefighters’ union, Local 718, which is run by his uncle Rich Paris, about the endorsement. “And, you know, John and I don’t agree on everything but I think that’s what I like about John,” Collins said. “He listens. He’s willing to take input and I believe that we’ll have a great working relationship when John’s mayor.”