Associated Press | The union representing mechanics and other ground workers at American Airlines is asking federal officials to let them take a big step closer toward a possible strike against the nation’s second-largest carrier.

The Transport Workers Union said Thursday they asked federal mediators to declare a deadlock in their negotiations over a new contract.

If mediators grant the union’s wish, and if either side rejects binding arbitration, it could start a 30-day “cooling-off” period after which the workers could strike.

American said it expects a federal mediator to instead order both sides into more bargaining sessions.

Company spokeswoman Missy Latham said the union’s demand undermined chances for a negotiated deal. She said the two sides “made significant progress” in talks this week and agree on 87 percent of contract items.

Union official John M. Conley shot back, “Unfortunately, one of the items we didn’t settle on was compensation.”

American said it has offered workers lump-sum payments in the first six months after a deal and 2.5 percent raises 18 months later, plus increases in vacation, holiday and sick days. The union, according to information on its Web site, is asking that maintenance technicians get raises of 13.5 percent over three years, retroactive to 2008.

The company conceded the sides remained “far apart” on other items too, including the company’s demand to control retiree medical and other costs.

The union represents 28,000 workers at American and its regional arm, American Eagle. Some workers at Eagle have reached tentative agreements that face ratification votes.

Meanwhile, American is stuck in difficult talks with its other two unions. The flight attendants’ union plans to ask mediators next week to declare an impasse in their negotiations, and has scheduled a strike-authorization vote next month. Pilots made pay demands that the airline quickly rejected.

Federal law makes it hard for airline workers to strike. They can walk off the job only if the National Mediation Board declares negotiations deadlocked and one side refuses binding arbitration. Even then, the president can block a strike — President Bill Clinton ordered American’s pilots back to their jobs minutes after they struck in 1997.

During the Bush administration, the mediation board generally rejected airline union requests to end negotiations, but the unions are optimistic that the Obama administration will change course.