Three Senate Democrats on Monday filed the latest legal challenge to President Donald Trump’s appointment of Matthew Whitaker as acting attorney general, arguing that his appointment circumvents the Senate’s role in the confirmation process.

The constitutionality and lawfulness of Whitaker’s appointment have been under scrutiny since Trump appointed him on Nov. 7, after former attorney general Jeff Sessions — Whitaker’s former boss — was forced out. Legal scholars and former Justice Department officials have questioned whether a non-Senate-confirmed official such as Whitaker can serve as the nation’s top law enforcement officer, even on an acting basis, and judges are being asked to weigh in.

Legal challenges to Whitaker’s appointment have already been filed in cases before the US Supreme Court and in federal district court in Maryland. The underlying lawsuits in those cases predate Whitaker’s appointment, and the challengers argue that because Whitaker must be formally substituted for Sessions as the top Justice Department official in the proceedings, the courts need to decide if Whitaker is lawfully serving as acting attorney general.

The lawsuit filed Monday in the US District Court for the District of Columbia on behalf of Sens. Richard Blumenthal, Mazie Hirono, and Sheldon Whitehouse is the first case filed as a direct challenge to Whitaker’s appointment.

The Senate vets and confirms senior executive branch officials, including the US attorney general. Although there is a federal law, the Federal Vacancies Reform Act, that gives the president authority to appoint an acting attorney general, the senators argue that the move was unconstitutional under the Appointments Clause, which states that the president “shall nominate, and by and with the advice and consent of the Senate … officers of the United States.”

The senators argue that they’re in a unique position to challenge Whitaker’s appointment because of the role they play in the confirmation process.

“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,” lawyers for the senators from the Protect Democracy Project and Constitutional Accountability Center argued in the complaint.

This is the third case that Blumenthal, Hirono, and Whitehouse have brought against the Trump administration. They’re also suing the president over his refusal to divest from his businesses since taking office, arguing that it violates the Constitution’s prohibition on officials accepting foreign “emoluments,” as well as for records related to Justice Brett Kavanaugh’s nomination. In the emoluments case, a federal judge found that the senators had standing to sue because under the Constitution, Congress was supposed to get a say in whether to approve any emoluments.

The Justice Department’s Office of Legal Counsel issued a memo last week defending the constitutionality and lawfulness of Whitaker’s appointment. The office acknowledged that it was almost unprecedented for a non-Senate-confirmed official to serve as acting attorney general — there was one such appointment in 1866, according to OLC — but pointed to other examples of non-Senate-confirmed individuals serving in senior-level posts on an acting basis. Lawyers in the office concluded that the Federal Vacancies Reform Act applied to someone in Whitaker’s position.

“President Trump’s designation of Matt Whitaker as Acting Attorney General of the United States is lawful and comports with the Federal Vacancies Reform Act, the Appointments Clause of the U.S. Constitution, Supreme Court precedent, past Department of Justice opinions, and actions of U.S. Presidents, both Republican and Democrat,” Justice Department spokesperson Kerri Kupec said in a statement Monday. “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. To suggest otherwise is to ignore centuries of practice and precedent.”