By Sarah N. Lynch

WASHINGTON (Reuters) - Three Democratic senators launched a new legal challenge on Monday to President Donald Trump's naming of Matthew Whitaker as acting attorney general, accusing Trump of depriving the U.S. Senate of its constitutional power to act on such appointments.

The lawsuit, filed by Senators Richard Blumenthal, Sheldon Whitehouse and Senator Mazie Hirono, asks a federal court in Washington to deem the appointment unconstitutional and bar Whitaker from serving as the top U.S. law enforcement official. The senators are members of the Senate Judiciary Committee, which customarily reviews attorney general nominations.

Congressional Democrats have expressed concern that Whitaker, a Trump loyalist, could undermine or even fire Special Counsel Robert Mueller, whose investigation of Russia's role in the 2016 U.S. election has cast a cloud over Trump's presidency.

"Americans prize a system of checks and balances, which President Trump's dictatorial appointment betrays," Blumenthal said in a statement.

Other legal challenges to Whitaker's appointment were brought last week by Maryland's Democratic attorney general and by a plaintiff in a gun rights case before the U.S. Supreme Court.

The senators' lawsuit said Trump violated the so-called Appointments Clause of the U.S. Constitution because the job of attorney general is a "principal officer" who must be appointed by the president and confirmed by the Senate. Whitaker is not a Senate-confirmed official.

It also accused Trump of violating a federal law that establishes the line of succession if the attorney general post becomes vacant, giving full authority to the deputy attorney general. Whitaker took over supervision of Mueller's investigation from Deputy Attorney General Rod Rosenstein when Trump appointed him on Nov. 7 as acting attorney general to replace Jeff Sessions, who the president ousted.

The senators argued that Rosenstein, a Senate-confirmed Trump appointee who installed Mueller as special counsel in May 2017, should be the acting attorney general until a Sessions replacement is nominated by the president and confirmed by the Senate.

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Whitaker in the past criticized the scope of Mueller's probe and talked about undermining it by slashing the special counsel's funding. Whitaker has not recused himself from overseeing the probe despite Democratic calls for him to do so.

'DO WHAT'S RIGHT'

Trump said in an interview on the "Fox News Sunday" program he would not intervene if Whitaker moved to curtail Mueller's investigation, and that his appointee is "going to do what's right." Trump, who said on Friday he had completed written answers to provide Mueller in the probe, has called the investigation a "witch hunt."

The senators' lawsuit cited Whitaker's criticism of Mueller's probe and his ties to World Patent Marketing, a company accused by the government of bilking millions of dollars from consumers.

"The U.S. Senate has not consented to Mr. Whitaker serving in any office within the federal government," it stated.

The Senate, it added, needs an opportunity "to consider his espoused legal views, his affiliation with a company that is under criminal investigation for defrauding consumers, and his public comments criticizing and proposing to curtail ongoing DOJ (Department of Justice) investigations that implicate the President."

The Justice Department last week issued a 20-page legal opinion calling Trump's appointment of Whitaker, who had been chief of staff to Sessions, lawful under a 1998 law called the Federal Vacancies Reform Act. Many legal experts have disagreed with the department's view.

The department cited only one instance in U.S. history - in 1866 - in which a non-Senate confirmed person was named acting attorney general. Its legal opinion also relied on an 1898 Supreme Court ruling that a so-called inferior officer is legally able to perform the duties of a principal officer "for a limited time and under special and temporary conditions" without needing Senate confirmation.

The Democratic lawsuit rejected that view, saying it fails to explain why Whitaker's appointment qualifies as "special or exigent."

Justice Department spokeswoman Kerri Kupec issued a statement defending Trump's designation of Whitaker as lawful, saying it comports with the Federal Vacancies Reform Act, the Constitution, Supreme Court precedent, past department opinions and actions of presidents from both parties.

(Reporting by Sarah N. Lynch; Editing by Will Dunham)