Attorney General Barbara Underwood on Monday filed a friend-of-the-court brief opposing the appointment of Matthew Whitaker as acting U.S. attorney general by President Donald Trump.

The brief is part of a Maryland case challenging the recess appointment by Trump being brought by a coalition of 15 attorneys general, including Underwood.

“The law is clear – and Matthew Whitaker’s appointment as Acting Attorney General is illegal, violating long-standing rules,” Underwood said in a statement. “Our coalition of Attorneys General will continue to do what’s necessary to protect the rule of law.”

The brief is in support of a motion that seeks to block Whitaker from exercising authority as the U.S. attorney general or to substitute deputy AG Rod Rosenstein as the defendant in an ongoing case between Maryland and the federal government over the Affordable Care Act.

The brief, along with New York, was backed by attorneys general in Pennsylvania, the District of Columbia, Connecticut, Delaware, Hawaii, Illinois, Maine, Massachusetts, New Mexico, North Carolina, Oregon, Rhode Island, Virginia, and Washington.