Portside

The American Federation of Teachers confirmed Friday that it was the donor behind One Boston, a mysterious political action committee that paid for a $480,000 television commercial supporting Mayor-elect Martin J. Walsh during the final days of the Boston mayoral race.

The national teachers’ union exploited discrepancies in state-by-state campaign spending disclosure laws to anonymously fund nearly a half million dollars worth of advertising on behalf of Walsh.

No information was disclosed at the time about One Boston’s ideological loyalties or donors, enraging government watchdog groups who had already been critical of the unprecedented amount of outside money that was flowing into the race, which ultimately became the state’s most expensive municipal race ever.

“In the last days of the campaign, we were solicited by an independent expenditure campaign and decided it was the quickest and most efficient way for us to ensure that working families had a voice in deciding Boston’s next mayor,” Michael Powell, an AFT spokesman, said in a statement provided to the Globe.

“That contribution funded a positive TV ad in support of Walsh,” he said. “We stand by the ad, and, while we leave it to others to decide, we think the ad made a difference in helping to elect Marty Walsh.”

Several members of the federation’s national leadership were outspoken in their support of Walsh and their dislike for his opponent, Councilor at Large John R. Connolly, who had several high-profile spats with the Boston Teachers Union in his six years on the council.

“The AFT is functionally the BTU,” Connolly said, reacting to the disclosure. “As a former candidate, I’m moving past the race. As a Boston public schools parent, I’m really angry that our teacher’s union would spend $500,000 on a mayor’s race and not attach their name to the contribution.”

Though it had vowed to remain uninvolved in the mayoral final, the Boston Teachers Union, which is formally affiliated with the AFT, endorsed Walsh on election day.

The Massachusetts database of campaign donations shows the AFT never gave money directly to Walsh’s campaign, which would have been subject to state-imposed contribution limits, or to One Boston, which would have been subject to disclosure requirements.

Instead, the national teacher’s union gave $480,000 to One New Jersey, a political action committee that has vocally opposed candidates who clash with teachers unions, most notably Governor Chris Christie of New Jersey.

In a complicated series of transactions, the AFT first gave the money to One New Jersey, which is not required under New Jersey campaign finance laws to disclose its donors. Then, One New Jersey gave the money to One Boston, a Massachusetts political action committee it set up for the purpose of funding an advertisement on behalf of Walsh. One Boston used the money for the pro-Walsh television commercial.

“We share the same values as Marty Walsh; that’s why we produced this positive, issues-oriented commercial,” said Joshua Henne, a spokesman for One New Jersey.

“Boston’s middle-class and working families are fortunate to have a mayor like Marty Walsh on their side,” Henne said.

The sudden appearance of One Boston-funded ads during the race’s final week without disclosure of the money’s origins outraged government watchdog groups and prompted calls from both candidates for the group to identity its donors.

“Transparency was a centerpiece of the campaign, and Mayor-elect Walsh was very clear in public from the beginning that all independent expenditures should voluntarily disclose their donors,” Kate Norton, Walsh’s spokeswoman, said in a statement provided to the Globe Fridayevening.

“The law prohibits any coordination between the campaign and any independent expenditure,” she said in the statement. “We don’t have any control over or awareness of their plans. Mayor-elect Walsh urged disclosure through statements to the press and sought to lead by example in providing complete transparency of his record, background, and contributions.”

The only name listed in documents associated with the group is Jocelyn Hutt, 55, a woman from Roslindale who, city records show, had not voted in three of Boston’s past four municipal elections. It remains unclear what role Hutt played in setting up One Boston or if she is linked formally to One New Jersey.

One Boston was one of three outside groups that spent heavily on behalf of Walsh, whose mayoral victory was in part buoyed by independent expenditures from labor-affiliated groups.

Working America, the political arm of the AFL-CIO, put more than $665,000 into the race on behalf of Walsh. Another group, Virginia-based American Working Families, has disclosed that the more than $1.2 million it spent on Walsh’s behalf came from labor unions.

State law allows political action committees that spent money on behalf of the candidates to wait until January, more than two months after voters cast ballots, to disclose the source of that cash.

In total, $2.5 million was spent via independent expenditures on Walsh’s behalf, compared with $1.3 million spent on Connolly’s behalf, exclusively from national education reform groups.