BOSTON - Boston's unions will have a mighty sharp arrow in their collective quiver as they sit down with city officials to renegotiate their labor contracts over the next 18 months: the threat of embarrassment for Mayor Thomas M. Menino during what should be his national triumph - the 2004 Democratic National Convention.

Contracts for more than 10,000 city employees will be up for renewal between now and the July 2004 convention. About 20 contracts, including those with the police, have expired. Some previous battles have stretched into years. And at least some union representatives are suggesting that they will use the convention to hold the mayor's feet to the fire if talks do not go smoothly.

"Do I think the convention is the key to our success? No," said Thomas Nee of the Boston Police Patrolmen's Association. "But it's definitely going to assure me I don't get killed in the process."

State AFL-CIO President Robert J. Haynes also hinted that failure by the city to provide union members with satisfactory contracts might have unwelcome consequences during the weeklong event.

"In the spirit of making sure the convention goes smoothly, the Massachusetts AFL-CIO wants to make sure that numerous unsigned contracts with the City of Boston are resolved to the satisfaction of our affiliate unions," Haynes wrote in a statement. "Resolving those unsigned contracts will go a long way toward ensuring a smooth 2004 Democratic Convention."

Labor relations are a sensitive issue for the mayor, who emerged from a protracted, disruptive and politically bruising battle with Boston's firefighters last year, after which he was criticized for handing out an overly generous contract to mollify the union. And, as he prepares to be the host to the convention, city finances are in the worst shape of his tenure, leaving him with little to offer the other powerful unions.

In addition, Menino is determined to show the nation that Boston has overcome its old reputation for political warring and gridlock, an effort that labor unrest would sabotage.

City officials said they are determined to complete all the contract negotiations well before the convention.