John Kerry does something right: Mr. Kerry had planned to give a speech here on Monday morning to the United States Conference of Mayors. But members of the city's largest police union, who have been working without a contract for two years, along with the firefighters, who are also in contract talks, have been picketing Mayor Thomas M. Menino, the host of the conference, wherever he goes, and were set to do so Monday at the hotel where Mr. Kerry was scheduled to speak. With each side hoping to press Mr. Kerry to embarrass the other, his schedule remained up in the air for more than 24 hours as he debated whether to antagonize a crucial Democratic supporter or a union local and its - not to mention his - allies in organized labor, a core Democratic constituency. By 8 p.m. Sunday, despite what one participant described as "tedious" efforts to broker a solution, Mr. Kerry's aides gave up hope that a deal could be reached to get the unions to suspend their picketing long enough for Mr. Kerry to attend the conference without having to cross the line. "We know that people on both sides have been working in good faith to resolve this situation," said Michael Meehan, a Kerry spokesman, in announcing the senator's cancellation of his speech. "We hope that they will redouble their efforts to find a resolution to this situation." Seth Gitell, Mr. Menino's spokesman, said the mayor was disappointed by Mr. Kerry's decision. Later Sunday night, after attending Mass and receiving communion at St. Vincent's Waterfront Chapel overlooking Boston Harbor, Mr. Kerry was asked how he would respond to the mayor. "I don't cross picket lines," he said. "I never have.''