Three Senate Democrats filed a federal lawsuit Monday asking a judge to oust acting Attorney General Matthew G. Whitaker and install Deputy Attorney General Rod Rosenstein instead.

Sens. Richard Blumenthal, Sheldon Whitehouse and Mazie K. Hirono, all members of the Judiciary Committee, said President Trump circumvented both the law and the Constitution by designating Mr. Whitaker.

They argue that undermines the Constitution’s demand that the president only appoint top officials with the “advice and consent” of the Senate.

“The U.S. Senate has not consented to Mr. Whitaker serving in any office within the federal government, let alone the highest office of the DOJ,” the senators said.

For Mr. Blumenthal, it’s his third major lawsuit against Mr. Trump and the administration, following previous complaints over the president’s business empire and the Constitution’s emoluments clauses, and another over the way documents were handled during the confirmation of Supreme Court Justice Brett M. Kavanaugh.

In the case of Mr. Whitaker, he was serving as chief of staff when Attorney General Jeff Sessions was ousted the day after the midterm elections. Mr. Trump designated Mr. Whitaker to assume the post in an acting fashion, leapfrogging Mr. Rosenstein and others who had gone through Senate confirmation.

Democrats fear Mr. Whitaker will hinder the ongoing special counsel’s investigation into the 2016 elections, Russian meddling and Trump campaign figures’ behavior.

In their lawsuit, the senators say if they’d had a chance to vet Mr. Whitaker, they would have prodded him on his views on the special counsel.

Mr. Trump said in an interview with “Fox News Sunday” that he was giving Mr. Whitaker free rein in oversight of the special counsel.

“The prospect that a president’s high-level executive branch appointments would be influenced by personal, rather than public, interests is one key reason why the nation’s Founders required such appointees to receive the advice and consent of the Senate in the first place,” said Elizabeth Wydra, president of Constitutional Accountability Center, which is assisting with the senators’ lawsuit.

The Justice Department’s Office of Legal Counsel, the government’s chief internal voice for legal matters, has opined that the Whitaker appointment is both legal and constitutional.

The OLC pointed to a number of precedents for acting principal officers to be designated in the same way as Mr. Whitaker, including leapfrogging other Senate-confirmed positions.

But the OLC found just one precedent, from 1866, for a non-Senate confirmed person to serve as acting attorney general.