The American Federation of Government Employees (AFGE) union released a statement on Monday announcing its lawsuit as the shutdown stretches past a week.
The lawsuit, filed in the U.S. Court of Federal Claims, alleges that the federal government is in violation of the law by requiring some federal workers, many of which are represented by the union, to work without pay during the shutdown.
"Our members put their lives on the line to keep our country safe," J. David Cox Sr., president of AFGE, said in the release. "Requiring them to work without pay is nothing short of inhumane. Positions that are considered 'essential' during a government shutdown are some of the most dangerous jobs in the federal government."
AFGE also denounced the shutdown earlier this month.
The partial shutdown began on Dec. 22 after the House and Senate failed to agree on an end-of-year government spending bill.
Democrats and Republicans are at odds over President Trump's demands for $5 billion to fund a wall at the U.S.-Mexico border. He has said he will not sign a spending bill that does not include his desired border wall funding.