A group of Senate Democrats is suing to block Matt Whitaker from serving as acting attorney general on grounds that his placement in the post was unconstitutional.

The suit, which is being filed by Sens. Richard Blumenthal (D-CT), Sheldon Whitehouse (D-RI), and Mazie K. Hirono (D-HI) in U.S. District Court for the District of Columbia, is the latest and most aggressive salvo against the Whitaker appointment. Last week, the Department of Justice Office of Legal Counsel defended Whitaker’s promotion in a memo that drew immediate criticism for its expansive understanding of the president’s power. That view is in hot dispute, including from the state of Maryland, which petitioned a federal judge to stop him from serving on constitutional grounds.

The latest suit, which was brought by the groups Protect Democracy and the Constitutional Accountability Center for the Senators, argues that Whitaker’s appointment violates the Constitution’s Appointments Clause because the U.S. Senate did not confirm him to his prior post. Whitaker was chief of staff to now-former Attorney General Jeff Sessions before President Trump elevated him to his current gig. Trump did so through the Vacancies Reform Act, which allows the staffing of vacant positions for up to 210 days. But many constitutional scholars have argued that the Vacancies Reform Act doesn’t let the president appoint people to cabinet-level positions who haven’t been senate confirmed. The Senate confirmed Whitaker in 2004 as a U.S. Attorney in Iowa, but his opponents—most prominently George Conway, the husband of White House senior staffer Kellyanne Conway, and former Solicitor General Neal Katyal—say that confirmation has effectively lapsed.

“Installing Matthew Whitaker so flagrantly defies constitutional law that any viewer of Schoolhouse Rock would recognize it,” Blumenthal said in a statement. “President Trump is denying Senators our constitutional obligation and opportunity to do our job: scrutinizing the nomination of our nation’s top law enforcement official. The reason is simple: Whitaker would never pass the advice and consent test. In selecting a so-called “constitutional nobody” and thwarting every Senator’s constitutional duty, Trump leaves us no choice but to seek recourse through the courts.”

Whitaker has publicly criticized Special Counsel Robert Mueller’s investigation into Russian meddling in the 2016 election. A few months after Mueller’s installation, Whitaker stated definitively that there was no collusion between Team Trump and the Kremlin. As a vociferous defender of the Trump administration, his appointment has spooked career Justice Department officials.

To legally challenge Whitaker’s appointment, the plaintiffs will have to show they have standing––meaning, that his appointment violated their rights. The question of whether this select group of Democratic senators has standing on grounds that Whitaker’s appointment violated their constitutional right to advise and consent the president on cabinet-level appointments will undoubtedly be debated. David Rivkin, a constitutional lawyer who served in the George H. W. Bush and Ronald Reagan administrations, told The Daily Beast previously that to win this kind of suit, the Senate would first have to vote to claim standing as an institution.

"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,” said Whitehouse in a statement. “Unless the courts intercede, this troubling move creates a plain road map for persistent and deliberate evasion by the executive branch of the Senate's constitutionally mandated advice and consent. Indeed, this appointment appears planned to accomplish that goal.”

President Trump has steadfastly defended Whitaker’s appointment and his fitness to oversee the Mueller probe.

“What do you do when a person’s right?” Trump told Fox News in an interview that aired Sunday. “There is no collusion. He happened to be right. I mean, he said it. So if he said there is collusion, I’m supposed to be taking somebody that says there is? Because then I wouldn’t take him for two reasons, but the number one reason is the fact that he would have been wrong. If he said that there’s no collusion, he’s right.”