Alaska Airlines flight attendants seeking a new contract with pay increases and better benefits have voted overwhelmingly to authorize a strike.

The announcement came during a “national day of action,” at which flight attendants from three separate unions picketed outside 30 U.S. airports, including Portland International Airport. The Association of Flight Attendants-CWA, which represents the Alaska flight attendants, said the strike was authorized with a 99% “yes” vote.

A strike is not imminent, and federal law makes it difficult for airline unions to conduct legal strikes. Any strike would require approval from mediators and, if approved, would be preceded by a 30-day “cooling off” period.

Mediators have already turned down one request by flight attendants at American Airlines to begin a countdown to a strike.

If it came to a strike, though, the dispute could be enormously disruptive.

Flight attendant unions have adopted a strategy they call “Creating Havoc Around Our System,” or CHAOS, in which the flight attendants walk out on individual flights or locations without warning. Resulting delays can ripple throughout the air travel network or force airlines into expensive contingency plans.

“There’s no excuse: Alaska management has the money to buy another airline, they certainly have the money to invest in Flight Attendants,” Jeffrey Peterson, the flight attendants union Alaska unit president, said in a statement.

“We have a simple message for management: Pay us, or CHAOS!”

Alaska Airlines in December announced plans to pay $1.9 billion for Hawaiian Airlines, an effort to grow the company into a competitor to the nation’s largest airlines. It acquired Virgin America in 2016.

The airline said it had offered a 15% immediate pay raise in addition to annual raises under a new contract. It said negotiators continue to meet and make progress.

“We agree with our flight attendants that we need a new contract, which is why we’ve been working hard to get an agreement,” the airline said. “We remain optimistic in the negotiations process.”

The union says the raises offered are effectively negated by high inflation.

Flight attendants also want to be paid during the time that passengers board the plane.

Only Delta Air Lines, whose attendants are nonunion, pays during boarding. At other U.S. carriers, hourly pay for crews starts when the passengers are seated and the plane doors close.

Alaska is the largest airline at Portland International Airport, accounting for nearly a third of the passengers that passed through the airport last year. The airline said Monday it was adding 20 daily departures from PDX this summer.