Three U.S. Senate Democrats sued to block President Donald Trump’s appointment of Matthew Whitaker as acting attorney general.

Their complaint, filed Monday in federal court in Washington, is at least the fourth legal challenge to Whitaker’s selection to temporarily replace Attorney General Jeff Sessions, whom Trump ousted this month, and the first specifically filed to dislodge Whitaker. Some Senate Republicans have already urged the president to nominate a permanent successor.

At the center of the suit, filed by Richard Blumenthal of Connecticut, Sheldon Whitehouse of Rhode Island and Mazie Hirono of Hawaii, is a constitutional provision giving them and their colleagues the “advice and consent” power to withhold approval of high-level presidential nominees including cabinet members and federal judges. Whitaker as the nation’s top law enforcement official “would never pass the advice and consent test,” Blumenthal said in a statement.

The lawmakers are seeking an order blocking Whitaker from performing the functions and duties of the office of the attorney general because he wasn’t confirmed by the Senate.

“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 the statement. He was referring to Special Counsel Robert Mueller’s probe into Russian interference in the 2016 presidential election and any role the Trump campaign may have played in those activities.

Justice Department spokeswoman Kerri Kupec declined to comment on the lawsuit but has disputed the basis of challenges to the appointment.

“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,” Kupec said in a statement Friday.

The Justice Department’s Office of Legal Counsel last week issued a memorandum stating that Whitaker’s appointment was legally valid.

Thousands more challenges could crop up, said Josh Blackman, a professor of constitutional law at the South Texas College of Law, in Houston.

“Because the attorney general oversees virtually every aspect of the Department of Justice, thousands of litigants throughout the country will be able to challenge his appointment,” Blackman said. “Sooner or later a federal court somewhere will declare Whitaker’s appointment unconstitutional. At that point, the Supreme Court will have to intervene to resolve the issue.”

Kupec was responding to a filing last week by Washington-based Supreme Court litigator Tom Goldstein that asked the justices to remove Whitaker’s name from a case where he is listed as acting attorney general, in favor of Deputy Attorney General Rod Rosenstein. Goldstein asserts Whitaker’s appointment violated federal law as well as the Constitution.

The issue has also been raised in a Maryland federal court lawsuit defending Obamacare filed by that state’s Democratic attorney general, Brian Frosh and, separately, in a St. Louis federal court.

Whitaker quickly emerged as a polarizing figure after his elevation. He had previously criticized the scope of the Mueller probe, over which he now holds supervisory authority, has questioned the power of the U.S. Supreme Court to invalidate legislation passed by Congress and, when seeking the Republican nomination for U.S. Senate in Iowa, opined that judges should view justice through the lens of the New Testament.

“President Trump has shown utter disregard for the bedrock constitutional plan for top executive branch officials to receive the advice and consent of the Senate before taking office,” said Elizabeth Wydra, president of the Constitutional Accountability Center, one of two public interest groups litigating the challenge.

Whitaker previously served as Sessions’s chief of staff, a post that didn’t require Senate confirmation.

The case is Blumenthal v. Whitaker, 18-cv-2664, U.S. District Court, District of Columbia (Washington).