State Representative Martin J. Walsh dominated fund-raising in the Boston race for mayor in August, collecting $383,000 — the biggest monthly haul of any of the dozen candidates to date — as an outside group prepares to fund additional television ads on his behalf.
A political action committee called American Working Families is spending $175,200 for two television ads promoting Walsh, its chairman said on Friday. After weeks in which the rival mayoral campaigns sparred over whether they should discourage spending by outside interests — and Councilor John R. Connolly told an advocacy group he did not want $500,000 spent on his behalf — Walsh voiced no reservations about the outside boost.
Instead, Walsh advisers said that about three-quarters of his contributions have come from individual supporters rather than interest groups and that his growing financial support demonstrates momentum.
“People are excited. I think it shows some traction,” said Joyce Linehan, a Walsh political consultant.
The other leaders in the race for campaign cash continued to run strong, too. Connolly came in second, as he did in July, with his campaign reporting contributions of $245,077 in August. Suffolk District Attorney Daniel F. Conley raised about $152,000, followed by City Councilor Michael Ross, who received approximately $126,248.
Final campaign finance data for August will be posted to the state Office of Campaign and Political Finance website in the coming days, but several leading campaigns were able to report their estimated contributions to the Globe in advance of the Aug. 31 deadline.
Twelve candidates are running in the Sept. 24 preliminary election for mayor. The two leading vote-getters will face off in the final election on Nov. 5.
While Walsh’s monthly fund-raising surged, he has less cash on hand — about $700,000 — than Conley, who is sitting on a war chest of $1.125 million. Connolly has about $589,000 left, and Ross’s campaign reported $435,000 cash on hand.
Walsh, a former laborer and union official, rankled some of his rivals by benefiting from significant support from unions. In August alone, union contributions boosted his campaign by at least $93,000 — more than some of his rivals in the race for mayor collected from all their donors in a month.
Unlike individuals, who can donate only $500 per election cycle in city races, unions can contribute up to $15,500 to a candidate.
That led some rival campaigns to point out the disparity and urge Walsh to forgo union money.
“We’re proud that Dan has raised over $800,000 in individual, itemized, publicly reported contributions and we call on Marty Walsh to level the playing field and disavow special interest money and sign the people’s pledge,” said Mike Sherry, a Conley spokesman.
City Councilor and mayoral candidate Rob Consalvo has pushed his fellow candidates to sign a “Boston pledge” agreeing to discourage campaign spending by outside groups.
Based on the People’s Pledge forged by US Senate candidates Elizabeth Warren and Scott Brown in 2012, the pledge would discourage PACs from spending for candidates by requiring each candidate to match any outside spending with a contribution to The One Fund Boston, which benefits victims of the Boston Marathon bombing.
Conley recently agreed to a similar pledge, and Connolly said he did not want Stand for Children, an educational advocacy group that endorsed him, to spend $500,000 on campaigning for him.
Independent expenditures from advocacy groups and political action committees help supplement spending and advertising by the campaigns.
But the chairman of American Working Families that is now backing Walsh blasted such pledges.
“At this point asking an organization to abide by a campaign pledge that limits their right to permissible political speech is arbitrary, unfair, and politically motivated,” Bud Jackson said in a statement. “It reinforces that these pledges are unsustainable gimmicks that can be manipulated, timed, and exploited for political purposes.”
His organization plans to air two 30-second TV ads for Walsh beginning next week.
One of the ads shows diverse constituents in a variety of workplaces and Boston streetscapes saying, “We are Boston.” The other focuses on one woman who says she “wouldn’t have this job if it weren’t for Marty Walsh,” and credits him for creating a union program that recruits women and minorities for trade jobs.
American Working Families, which backs progressive public policy, describes itself on its website as dedicated to “fighting back against the war on working people.”
Consalvo spokesman Kevin Franck countered: “I would expect that a super PAC from outside Virginia would think limiting outside money in the mayor’s race is a bad idea. But the voters of Boston think they should decide who the next mayor is.”
Another super PAC called Working Families reported spending more than $230,600 to benefit Walsh’s campaign. Stand for Children reported spending more than $8,800 for Connolly’s campaign, which also benefited from about $50,000 from a second education advocacy group called Democrats for Education Reform.
Consalvo reported raising $117,000 in August — all from individuals, Franck noted — leaving his campaign with about $56,000 in cash on hand. Nonprofit executive John Barros reported $40,548 for the month of August and just under $70,000 cash on hand for the campaign. Other campaigns were not able to report their numbers for the month.Stephanie Ebbert can be reached at Stephanie.Ebbert@globe.com. Follow her on Twitter @stephanieebbert.