Mayoral candidate John Connolly on Wednesday publicly denounced $500,000 pledged to him earlier this week by a national education nonprofit and became the third candidate in the race to swear off special-interest money.
"I did not ask for any money from outside groups," said Connolly, a city councilor, who was harshly criticized by his opponents after the pledge was reported this week. "And I don't want it."
Following Connolly's announcement, Stand for Children, an Oregon-based education advocacy group, said it would honor his wishes and not spend any money in support of the candidate during the preliminary contest in Boston's mayoral race.
The financial pledge, reported Tuesday, was believed to be one of the largest monetary commitments to a Boston municipal candidate ever made by an outside special-interest group.
While federal law prohibits candidates from coordinating with outside groups spending money on their behalf, the promise of that degree of support for advertising and other promotional efforts seemed to give Connolly — already a formidable force in the 12-person race — another powerful tool.
His opponents wasted no time lining up against him, saying he was sullying the integrity of the race by allowing an outside group to spend so much money on his behalf. On Wednesday, one of his harshest critics, Suffolk District Attorney Daniel F. Conley, applauded Connolly.
"John did the right thing today," said Conley, who had himself sought Stand for Children's endorsement. "He knew this was wrong, and he did the right thing today."
Multiple candidates sought an endorsement from Stand for Children, an at-times controversial group that advocates for longer school days, charter schools, and expanding prekindergarten access.
The group has come under fire from some on the left for lobbying in favor of legislation that they say would undercut teachers unions and for taking large donations from entities with corporate ties, including the Walton Family Foundation.
By law, Connolly had no power to stop the group from spending money in support of his candidacy, but on Wednesday, just days after pledging to loosen its purse strings, the group said it would not advertise on his behalf.
"We remain excited about his candidacy and his enthusiastic support for better schools in Boston," Jason Williams, executive director of Stand for Children Massachusetts, said in a statement to the Globe. "We will focus all of our energy during this preliminary campaign on the issues that are central to our mission."
Political observers believe Connolly's derailment of the group's plans to spend big in his support could hamper his efforts to match the union money that state Representative Martin J. Walsh has or the massive war chest amassed by Conley during his years in office.
Even Connolly acknowledged that swearing off the money could undercut his ability to get his message out as broadly as opponents who make no commitment to discourage outside spending.
At the news conference, Connolly stopped short of saying he would sign the so-called Boston Pledge, which he called a "political gimmick." The pledge, championed by city councilor and mayoral candidate Rob Consalvo, would require candidates to offset any outside money spent on their behalf by giving a matching donation to the One Fund, which benefits victims of the Marathon bombings.
But hours later, Connolly vowed to sign onto the pledge. Conley has also signed the pledge, which is designed to limit outside expenditures.
The pledge is modeled loosely on a similar agreement signed by US Senate candidates Scott Brown and Elizabeth Warren in 2012.
"I hope that the nine remaining candidates will sign it, there's still time," Consalvo said. "It's a proven model that will ensure a clean, transparent election."
But any hope that Walsh would swear off outside money was soon dashed.
"That was a great bit of political theater this morning," Walsh said in a statement on Wednesday afternoon. Connolly "was right this morning when he said that the pledge was nothing more than a political gimmick. Unlike John Connolly, I felt that way this morning and I haven't changed my mind."
A spokeswoman for City Councilor Felix G. Arroyo, another candidate with strong ties to organized labor, said he would be open to signing the pledge, but raised questions about whether it would work without all candidates buying in.
"In order for it to be effective, all candidates must sign on, and it is not enforceable otherwise," said Heather Perez, a spokeswoman for Arroyo's campaign. Several other campaigns have privately expressed similar concerns.
Until this week, many of the campaigns had remained silent on the topic of outside money in the race, and ignored Consalvo's letter asking them to sign the pledge as well as requests from the Globe that they clarify their stances.
Representatives for John Barros, Charles Yancey, Charles Clemons, and David James Wyatt did not respond to the Globe on Wednesday when asked whether they would commit to the Boston Pledge.
A spokeswoman for former state representative Charlotte Golar Richie did not respond to requests for comment, but the candidate — who has benefited from money spent on her behalf by Emily's List — has previously told the Globe that she would not sign the pledge.
On Wednesday, Golar Richie was joined by Codman Square Health Center founder Bill Walczak and City Councilor Mike Ross, who told the Globe they will not sign the pledge.
"I don't think any of this back-and-forth matters," Ross said. "Boston voters are smart and will pick the candidate with the best ideas."
Wesley Lowery can be reached at Wesley.Lowery@globe.com.