Massachusetts Gov. Mitt Romney (search) replaced Democratic presidential candidate John Kerry (search) as a speaker for the nation's mayors on Monday after Kerry canceled a speech rather than cross a police union picket line.

Romney, a Republican, met nearly 100 picketing police officers before entering the front door of the hotel where the U.S. Conference of Mayors (search) convened. A few of the protesters shouted taunts at the governor. The Boston Police Patrolmen's Association has been working without a contract for two years.

An ally of President Bush, Romney has campaigned in recent weeks for the president's re-election but has stayed out of the dispute between the police union and Boston Mayor Thomas Menino, a Democrat. The governor said he would speak to the group as a show of support for Menino.

"A mayor, a governor and a president have a responsibility for making tough decisions and balancing budgets. Senators don't," Romney said. "You always want to support labor and the efforts of labor ... but our first responsibility is to the people."

Menino had urged Kerry to attend the conference, which he called "an important event for urban America," and contended the police union's demonstrations did not constitute a legitimate picket line. However, Kerry said Sunday night: "I don't cross picket lines. I never have."

Menino said the unions are undermining Kerry and that he was disappointed with Kerry's decision. "I would think that he would come and talk to the mayors who are making a difference in America every day, who are on the front lines of the issues that face working people," Menino said.

Unions for police, firefighters and other city employees have been demonstrating throughout the mayor's meeting, seeking to shame Menino into settling their disputes. Labor groups plan major protests at some convention events, but police union spokesman Jim Barry said the union would not picket Kerry during the convention.

More than 200 mayors are attending the meeting of the conference, a nonpartisan organization that brings together mayors from cities with populations of more than 30,000 people.