(CNN) Three Democratic senators on Monday filed a lawsuit challenging the appointment of acting Attorney General Matt Whitaker, ratcheting up the court effort to declare his placement atop the Justice Department as unconstitutional.

Sens. Richard Blumenthal of Connecticut, Sheldon Whitehouse of Rhode Island and Mazie Hirono of Hawaii filed the suit in US District Court on Monday, represented by the public interest groups Protect Democracy and the Constitutional Accountability Center.

"The stakes are too high to allow the president to install an unconfirmed lackey to lead the Department of Justice — a lackey whose stated purpose, apparently, is undermining a major investigation into the president," Whitehouse said in a statement.

The lawsuit is only the latest challenge to Whitaker's appointment to replace Jeff Sessions after President Donald Trump fired his attorney general the day after the election.

Whitaker has come under fire from Democrats and others because he was a vocal critic of special counsel Robert Mueller's investigation before joining the Justice Department.