U.S. airlines will have to pay more than they originally anticipated in order to reach new contracts with pilots, who are using the increasing demand for pilots due to a shortage and rising travel to negotiate better wages.

On Wednesday, the Allied Pilots Association said its board voted 15-5 to reject an offer by American Airlines. This would provide staff with raises of 19% in three steps over two years.

American Airlines spokesman Dennis Tajer said that management’s priority is keeping pay increases as low as possible, and they’ve ignored crew-scheduling changes that the union claims will reduce the number of canceled and delayed flights.

“Management’s failure to invest in a pilot contract that meets passenger demand creates more uncertainty for the holiday travel season, and even next summer,” he said.

American Airlines, which has about 15,000 pilots and is based in Fort Worth, Texas, did not immediately respond to a request for comment.

Recently, the pilots at American Airlines rejected an offer for pay of up to 14.5% over 18 months. This mirrored the sentiment of Delta Air Line pilots this summer, as they had previously rejected a 19% offer by their employer. Both are represented by the same union: Air Line Pilots Association.

Taken together, developments show how labor is taking advantage of the situation at the nation’s three largest airlines: United, Delta and American. Smaller regional carriers such as Southwest Airlines and JetBlue are also feeling the ripple effect from the pilot shortages.

Numerous pilots have been picketing airports in recent months, but federal law prohibits airline workers from striking as long as federal mediators haven’t determined that more negotiations would be futile. Congress and the President may intervene to stop a walkout if this is the case.

In 2010, pilots at Spirit Airlines went on strike over their pay for several days before negotiators for the union and airline came to an agreement. In 1997, President Bill Clinton ordered American Airlines pilots to keep flying as a strike deadline passed.