A group of Senate Democrats sought to oust acting Attorney General Matthew Whitaker from his post in a lawsuit filed Monday in Washington, D.C., arguing that his appointment violates the Constitution.

Sens. Mazie Hirono, D-Hawaii, Richard Blumenthal, D-Conn., and Sheldon Whitehouse, D-R.I., argued that Whitaker, who took over following the resignation of Attorney General Jeff Sessions, should not be allowed to serve in the role. The senators, members of the Senate Judiciary Committee, said that his appointment circumvents the advise-and-consent clause of the Constitution meant to serve as a check on the president.

“Indeed, if allowed to stand, Mr. Whitaker’s appointment would create a road map for the evasion of the constitutionally prescribed Senate advice-and-consent role,” the complaint said.

Whitaker has lead the DOJ for less than two weeks, following Sessions’ resignation at President Donald Trump’s request the day after the midterm elections, and already attracted several legal challenges to the constitutionality of his appointment. The latest suit from the senators argued that the DOJ has its own succession statute — which designates the Deputy Attorney General as acting AG — that should trump the Federal Vacancies Reform Act under which Trump named Whitaker.

In a tweet Monday, Hirono argued that Trump’s appointment of Whitaker would “subvert the Constitution” if allowed to stand.

“We want the court to make clear that the Senate must confirm Matthew Whitaker’s appointment as acting Attorney General — otherwise this temporary appointment violates the Constitution’s Appointments Clause.” Hirono tweeted.

The DOJ has maintained that Whitaker’s appointment complied with the law, specifically the FVRA and the Constitution.

“There are over 160 instances in American history in which non-Senate confirmed persons performed, on a temporary basis, the duties of a Senate-confirmed position,” DOJ spokeswoman Kerri Kupec said in a statement. “To suggest otherwise is to ignore centuries of practice and precedent.”

The president’s allies on Capitol Hill have also come to the defense of putting Whitaker atop the Justice Department. Sen. Chuck Grassley, R-Iowa, the chair of the committee, has pushed back on the criticism of Whitaker and said Thursday he would prefer to hold confirmation hearings for President Donald Trump’s eventual new attorney general rather than have an oversight hearing for Whitaker.

Further, he said that Whitaker’s appointment was “in strict conformance with the law.”

The Democrats’ challenge to Whitaker’s legitimacy comes on the heels of another two legal challenges, one from the state of Maryland and another from an ex-convict seeking to overturn a federal ban on felon gun ownership.

Maryland, in a dispute with the Trump administration over the constitutionality of the Affordable Care Act’s individual mandate, made similar arguments to the Senators in their filing arguing that Whitaker’s appointment should be void. The argument stems from a New York Times op-ed filed by former Solicitor General Neal Katyal, currently a partner at Hogan Lovells.

Barry Michaels, who pled guilty to securities fraud in 1998, served 15 months in prison and now wants to buy a firearm, argued that Whitaker should not be named as the defendant in his case.

Trump’s appointment of Whitaker drew swift public concerns about the future of the investigation by Special Counsel Robert Mueller, as the decision passed over the current Deputy Attorney General Rod Rosenstein who supervises the probe. Whitaker wrote an op-ed for CNN in August 2017 that criticized the extent of the Mueller probe, saying it was “going too far,” and outlined a strategy to choke off funding for the investigation.

In his own responses to criticisms of the decision, Trump himself has drawn a comparison between Whitaker and Mueller, who he has publicly criticized for months.

“So funny to see little Adam Schitt (D-CA) talking about the fact that acting Attorney General Matt Whitaker was not approved by the Senate, but not mentioning the fact that Bob Mueller (who is highly conflicted) was not approved by the Senate!” Trump tweeted.

The Senators are represented by Anne Harden Tindall and Benjamin Leon Berwick of the Protect Democracy Project and Brian Rene Frazelle, Elizabeth Bonnie Wydra and Brianne Jenna Gorod of the Constitutional Accountability Center.

Counsel information for Trump and Whitaker were not immediately available.

The case is Blumenthal et al. v. Whitaker et al., case number 1:18-cv-02664, in the United States District Court for the District of Columbia.