Asked before boarding his campaign plane in Baltimore on Monday if he had any influence on the union's decision, Kerry responded with a wink and a chuckle, reports CBS News' Steve Chaggaris. Asked if he offered them something, Kerry said, "just diplomacy."
A Kerry campaign official said the senator's decision to cancel his speech was motivated in part by the expectation that the union would not protest at the FleetCenter during the late-July convention.
"The senator . . . kept his eye on the big prize, the convention, in order to achieve a trade-off with the unions," a Kerry spokesman said.
The union's change of heart allows Kerry and thousands of Democratic delegates, as well as thousands of journalists, to come and go at the convention site unimpeded, despite Boston's ongoing labor woes.
"I don't cross picket lines. I never have," Kerry said as he left Mass Sunday night at Our Lady of Good Voyage chapel in South Boston.
Boston Mayor Thomas Menino had called on Kerry to attend, saying the conference was "an important event for urban America," and that pickets set up by the Boston Police Patrolmen's Association and other union members did not constitute a legitimate picket line.
Menino has been locked in a battle with several city unions over unsettled contracts.
"We know that people on both sides have been working in good faith to resolve the situation, and we hope that they will redouble their efforts to find a resolution," said Kerry campaign spokesman David Wade.
Menino said that he spoke with Kerry on Sunday evening, and that he was "very, very disappointed" with Kerry's decision.
"I would think that he would come and talk to the mayors who are making a difference in America everyday, who are on the front lines of the issues that face working people," Menino said.
"We're very proud of the senator and his stand," said Jim Barry, a spokesman for the police union.
However, the police and firefighters are not actually on strike, so Menino has said their protests are not really picket lines.
Rhonda Spears, a spokeswoman for the mayors conference, said the "mayors are outraged" by Kerry's decision to cancel his appearance and Detroit Mayor Kwame M. Kilpatrick said he was disappointed and angry.
"This was the opportunity for Sen. John Kerry to give a message for how he's going to help mayors and cities," Kilpatrick said. "I'm concerned whether he's going to support the mayors of this country in delivering for their citizens."
According to an agenda, meetings during the mayors conference are covering issues like community policing, job creation, housing, children's health and sustainable development.
Boston police have been working without a contract for about two years. According to the Globe, they are seeking a 16 percent pay raise over four years. The city has offered 11.9 percent, the newspaper says.
Boston's firefighters are also staffing picket lines, apparently hoping that their pay raise will be as good as the cops'.
The union dispute is a major logistical obstacle — and political risk — for the Democratic convention that opens in Boston July 26.
A federal judge has warned pickets led by the Boston Police Patrolmen's Association to stop blocking people or vehicles from entering the job site at the FleetCenter, the site of the convention.
Earlier this month, the police union and its allies surrounded workers and trucks that have tried to enter the site, shouting at them to "go home" and physically standing in their way.