SECRETARY OF STATE William Galvin has not survived 43 years in the rough and tumble arena of Massachusetts politics by being meek. So when there’s a threat to his fiefdom, he does what most successful politicians do: He goes on the attack.

Galvin, facing a credible primary challenge for the first time in a dozen years and only the second time since he won the office in 1994, has over the last week made a big issue out of an Independent Expenditure Political Action Committee (IEPAC) that he claims was set up for the sole benefit of his challenger, Boston City Councilor Josh Zakim. Galvin has used the claim to try to force Zakim into signing a “People’s Pledge,” an agreement first used in the 2012 race between Scott Brown and Elizabeth Warren to keep outside money out of their US Senate race.

Independent Expenditure Political Action Committees are often associated with dark money, the term used for contributions to nonprofit organizations set up under Internal Revenue Service regulations that keep the identities of their donors secret. The committees are not affiliated with a candidate and cannot coordinate with a campaign but can spend money to support – or oppose – candidates and referendums.

Galvin’s only evidence for his dark money allegation is the fact Drew Beres, listed as the chairman and sole contact for the Forward in Mass IEPAC, has given money to Zakim in the past and hasn’t given money to any other Massachusetts politicians. The theory is that Beres must have set the committee up to help Zakim.

“There is no other person that could have benefitted,” said Galvin campaign spokeswoman Mara Dolan, repeating the connection her boss made during the only televised debate in the race on Tuesday. “The only person left is Josh Zakim.”

Zakim said the charge is bogus. While he admits he met Beres at a fundraiser in 2013, he said he has had no contact with him since.

Beres, a Chicago attorney who did not return multiple calls and emails for comment, did in fact contribute $200 to Zakim’s first campaign for city council in 2013, and a search of the state’s campaign finance database shows no other statewide candidate received money from him.

Beres, however, has given money to other Massachusetts pols. According to the Federal Election Commission, Beres donated $200 in 2013 to Joseph P. Kennedy III’s initial run for Congress. And in May  Beres made separate $500 donations to two PACs associated with US Rep. Seth Moulton. He also contributed $400 in March to Maura Sullivan, who is running in the Democratic primary for Congress in New Hampshire.

The IEPAC so far has only filed its organization form and, as of August 15, there are no records of expenditures or contributions.

The Galvin campaign dismissed the connection between Beres and Moulton saying federal and state campaigns are separate and one committee’s funds cannot be used to benefit the other.

To further the ties they see between Beres and Zakim, Galvin aides said employees in the secretary of state’s office have “heard” that Zakim has made a number of trips to Chicago, though they don’t claim he met with Beres.

A Zakim campaign spokesman said the allegations are baseless. “Josh met him [Beres] once at a fundraiser in 2013 during his first campaign,” campaign spokesman Jon Tapper said in an email. “Josh also said he’s been to Chicago twice in his life, not in at least the last five years. He’s never seen him there.”

Beres is an attorney at the law firm of Kirkland & Ellis, which has a Boston office. Dolan, the Galvin campaign spokeswoman, pointed out that the firm’s Boston website uses an image of the Zakim Bridge, named after the late civil rights activist Lenny Zakim, father of Josh Zakim. She seemed to suggest the bridge image showed Beres was somehow tied to Zakim. But the iconic span, officially named the Leonard P. Zakim Bunker Hill Bridge, is used on numerous displays, advertisements, and television shots of Boston and has come to be a familiar landmark for the city.

Zakim had initially balked at signing the pledge to refuse outside financial support, calling it a diversion by Galvin from his use of the office to pump up his campaign by putting out ads and videos with his image and name on them. But Wednesday Zakim offered his own version of the pledge and added a condition that the two meet in two more debates in addition to the next one on August 24, which is slated to be the last one. In a statement released by his campaign, Galvin said he’s “still reviewing” Zakim’s offer, but once again lambasted the councilor for not outright agreeing to the pledge.

“You shouldn’t have to sweeten the pot to get someone to do the right thing,” Galvin said about tying the debates to the pledge. “I have said unilaterally that under no conditions will I take or benefit from dark money…He should simply just say yes to taking the ‘People’s Pledge,’ but I am still reviewing the proposal.”

Campaign allegations with claims loosely tied to the truth are nothing new in any election. Zakim, for instance, claims Galvin was admonished by federal regulators in an audit by the Elections Assistance Commission for using his office resources to carry on an adjunct campaign. But the audit does not make that claim and Galvin has not been found in violation of any conflict of campaign law.