A group of 32 senators say federal mediators should speed up labor negotiations between airlines and their flight attendants and other workers, even granting them permission to go on strike "as necessary."

The lawmakers said Wednesday that airlines feel no pressure to reach contract agreements quickly because federal law makes it difficult for airline workers to strike. That causes talks to drag on for years, they said.

The senators -- 31 Democrats and independent Bernie Sanders of Vermont -- joined House Democrats by weighing in while flight attendants at American Airlines and United Airlines are trying to gain more leverage at the bargaining table.

Before they can legally strike, airline workers need permission from the National Mediation Board, which must determine that more negotiations are hopeless. The board has already turned down a request by American's flight attendants.

The lawmakers complained in a letter to the board that without new contracts, airline employees can go years without a raise, "while airline carriers make record-breaking profits." They asked the board to use all its powers, including granting permission to strike "as necessary," to settle long-running negotiations.

The senators said the ability to strike is a fundamental right that has helped workers in other industries win "groundbreaking new contracts in recent years."

Flight attendants at American, United and other airlines have tried strike-authorization votes and picketing at airports to put pressure on the carriers. The workers are frustrated that they haven't been able to win the large wage increases that pilots at Delta, United, American and Southwest have received under contracts approved since the start of last year.

The mediation board has not permitted an airline strike since 2010, when Spirit Airlines pilots conducted a brief walkout. Even with mediators' permission, strikes can be delayed or blocked by the president and Congress.