The largest federal employees union filed a lawsuit on Thursday seeking to block the Trump administration from ending collective bargaining for about 50,000 Transportation Security Administration agents that staff checkpoints at U.S. airports and other transportation hubs.

The lawsuit in Seattle federal court claims the Department of Homeland Security canceled a bargaining agreement covering TSA officers as retaliation against unions for challenging other Trump administration initiatives.

The lawsuit seeks to reinstate the seven-year agreement, which was enacted last year, and bar DHS from rescinding it again.