House Speaker Ron Mariano was supposed to put some political heft behind state Rep. Jon Santiago's campaign for mayor on Tuesday, but his trip to the South End to endorse the two-term Democrat turned into a major headache for the mayoral hopeful instead.

Mariano apologized Tuesday afternoon after making a quip during the event at Plaza Betances that he was "afraid my car's gonna get stolen," generating swift backlash on social media and a call from one other candidate for mayor - John Barros - for Santiago to reject the speaker's endorsement.

Mariano, a Quincy Democrat, was one of 16 lawmakers on hand to offer their support to Santiago, and his comments at the time drew chuckles from those standing behind the microphone, according to videos of the event posted online.

It didn't take long, however, for the sentiment to change as video capturing the audio was posted online and reports from the event became public.

"I'm sorry and I regret my comments. It was a poorly delivered attempt [at] humor referencing my personal experience as a college student in the 60s when my car was stolen, which I had shared with folks at the event prior to the program," said Mariano, a Quincy Democrat who attended nearby Northeastern University.

"My intention was not to portray the neighborhood or the city in a negative light, but to endorse the candidate who I believe should be the next mayor of Boston," he continued in a statement issued through his campaign.

Mariano had joined more than a dozen House members at the plaza in the middle of the Villa Victoria housing development in the South End to endorse Santiago, who lives in the South End. Prior to the press conference, people involved in the event said Mariano had walked around with Santiago to view the area, and commented on how the neighborhood had changed since he was a student.

Santiago, a Boston Medical Center emergency room physician, is one of six major candidates seeking to succeed Labor Secretary Marty Walsh - himself a former House member - full-time at City Hall, but he is the only member of the Legislature in the race.

He has previously been endorsed by a number of his Beacon Hill colleagues in the Boston delegation, including Ways and Means Chairman Aaron Michlewitz of the North End, Assistant Majority Leader Michael Moran of Brighton, and Reps. Kevin Honan of Allston, Ed Coppinger of West Roxbury and Dan Ryan of Charlestown.

Santiago called the speaker's comment's "disappointing," and a spokesman said the two men talked after the event.

"Shortly before our event, I was proudly showing him how much our community had changed since he was here as a student fifty years ago. He agreed. Friends are honest with each other and I've expressed my concern to him. He's said he's sorry and I accept the apology, but I reject the comment because it's emblematic of the kind of thinking we're trying to move beyond," Santiago said in a statement.

Before Mariano apologized and Santiago expressed his disappointment, Barros said Santiago's failure to call out the comment in the moment "speaks volumes to the lack of leadership we could expect from him if he were elected Mayor of Boston."

"Representative Santiago should decline the Speaker's endorsement and join me and the growing number of Bostonians who ask that the Speaker simply say he was wrong and he's sorry," said Barros, a two-time candidate for the mayor who led the city's economic development efforts under Mayor Walsh.

Santiago said he would not reject the speaker's endorsement.

"The Speaker is my friend and success in expanding opportunity for our city means bringing everyone to the table who wants to be a part of it and having tough conversations. He's apologized and I accept it and look forward to working with him as mayor to advance the city's needs on Beacon Hill," he said in a statement.

A recent poll conducted by MassINC Polling Group found Santiago tied with the city's economic development chief John Barros at the bottom of the mayoral field with 3 percent, but the Puerto Rico native polled near the top of the pack among Latino voters.

City Councilor Michelle Wu led the race, according to that poll, with 19 percent, just ahead of the 18 percent supporting Mayor Kim Janey, who stepped up from the City Council after Walsh resigned. Wu and Janey were trailed by City Councilors Annissa Eassaibi George at 6 percent and Andrea Campbell at 4 percent.

Majority Leader Claire Cronin, of Easton, joined Mariano in the South End to endorse Santiago, along with Speaker Pro Tempore Kate Hogan of Stow, Rep. Frank Moran of Lawrence, Rep. Jim O'Day of West Boylston, Rep. Sean Garballey of Arlington, Rep. Paul Donato of Medford, Rep. Tommy Vitolo of Brookline, Rep. Meg Kilcoyne of Northborough, Rep. Richard Haggerty of Woburn, Rep. Paul McMurtry of Dedham, Rep. Danielle Gregoire of Marlborough, Rep. William Driscoll of Milton and Rep. Jessica Giannino of Revere.

Photos from the event show that Reps. Paul Donato of Medford and Tommy Vitolo of Brookline were also in attendance.