The union representing Southwest Airlines Co.’s flight attendants voted to give leaders the ability to call a strike, a step toward a possible walkout if talks falter in federal mediation.

The authorization was approved by more than 98% of voting members of the Transport Workers Union of America Local 556, which represents about 21,000 Southwest flight attendants. Until a new contract is ratified, members are working under pay scales and rules that were part of a 2016 agreement.

While the vote doesn’t mean the union will strike immediately, it clears the way for a walkout if one is allowed by the National Mediation Board following a multiphase process outlined in the Railway Labor Act, which governs airline-union relations. Other unions have taken similar action to authorize strikes, including one representing Southwest’s pilots and another for American Airlines Group Inc.’s flight attendants.

The Southwest flight attendants union had a tentative deal in place that would have provided an immediate 20% pay increase along with annual boosts through 2028, but members voted it down last month by a wide margin.