Fifteen attorneys general filed an amicus brief on Monday supporting Maryland Attorney General Brian E. Frosh's motion to block Matthew Whitaker from serving as US attorney general.

Whitaker was named acting attorney general following former Attorney General Jeff Session's resignation in early November. Whitaker previously served as Session's chief of staff not as deputy attorney general.

The 15 attorneys general argue that Whitaker's appointment is illegal because it breaks the stated chain of succession.

On November 13, Frosh's office filed a motion for a preliminary injunction to stop Whitaker from acting as attorney general, or to substitute Deputy Attorney General Rod Rosenstein as a defendant in an ongoing suit between Maryland and the federal government over the Affordable Care Act.

Fifteen attorneys general filed an amicus brief on Monday supporting Maryland Attorney General Brian E. Frosh's motion to block Matthew Whitaker from serving as US attorney general.

Whitaker was named acting attorney general following former Attorney General Jeff Session's resignation in early November. Whitaker previously served as Session's chief of staff not as deputy attorney general.

In the amicus brief, attorneys general from the Pennsylvania, the District of Columbia, New York, Connecticut, Delaware, Hawaii, Illinois, Maine, Massachusetts, New Mexico, North Carolina, Oregon, Rhode Island, Virginia, and Washington argue that the appointment of Whitaker is illegal because it "ignores long-established vacancy succession laws, and is in violation of Congress’ clear designation of the Deputy Attorney General as the Acting Attorney General," according to a statement from New York State Attorney General Barbara Underwood.

On November 13, Frosh's office filed a motion for a preliminary injunction to stop Whitaker from acting as attorney general, or to substitute Deputy Attorney General Rod Rosenstein as a defendant in an ongoing suit between Maryland and the federal government over the Affordable Care Act.

Read more: Here's a full timeline of acting AG Matthew Whitaker's controversial past

Frosh argues that Whitaker's appointment violates violates 28 U.S.C. Section 508, which stipulates that the deputy AG should take over if the attorney general resigns, and that Whitaker's appointment violates law that stipulates that the Senate must confirm "principal officers" — of which attorney general was one of the original four stipulated in the Constitution.

"In this case, the health care of millions of Marylanders and Americans is at stake," Frosh said in a statement. "The suit cannot go forward without a legitimate Attorney General, and an Acting Attorney General making decisions that could affect matters of life and death without lawful authority puts all of us at risk."

The amicus brief from the 15 attorneys general supports Maryland, and argues that the legal uncertainty around Whitaker makes it difficult for the states to coordinate law enforcement agencies with the Department of Justice, and thus impacts the residents of the 14 states plus the District of Columbia.

The Department of Justice did not immediately respond to INSIDER's request for comment on Monday night.