About 3,200 union members at Boeing facilities in Michigan and Illinois went on strike at midnight on Monday after rejecting an contract offer from the company, the union said.

Local members of the International Association of Machinists and Aerospace Workers, who build and maintain fighter jets, including the F-15 and F/A-18 models, voted on Sunday to reject Boeing’s latest contract offer.

“IAM District 837 members build the aircraft and defense systems that keep our country safe,” IAM Midwest Territory General Vice President Sam Cicinelli said in a statement.

Cicinelli added, “They deserve nothing less than a contract that keeps their families secure and recognizes their unmatched expertise.”

The union members work at Boeing facilities in St. Loius and St. Charles, Missouri, along with Mascoutah, Illinois, according to the union.

They had voted on July 27 to reject an earlier 4-year contract proposal put forward by the company, the union said.

“We’re disappointed our employees rejected an offer that featured 40% average wage growth and resolved their primary issue on alternative work schedules,” Boeing said in a statement on Sunday.

Boeing added, “We are prepared for a strike and have fully implemented our contingency plan to ensure our non-striking workforce can continue supporting our customers.”