Richard Blumenthal of Connecticut, Mazie Hirono of Hawaii, and Sheldon Whitehouse of Rhode Island filed the suit on November 19, arguing that Whitaker's appointment violates the Constitution because he hasn't yet been confirmed by the Senate.
"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", Blumenthal said in a statement.
Kamenar's challenge to the legitimacy of the special counsel probe is based on a claim that Mueller's appointment was not handled properly.
This is a developing story. The Justice Department's Office of Legal Counsel issued a 20-page opinion last week arguing that Whitaker's appointment was legitimate, while conceding that the last time that the acting attorney general wasn't Senate-confirmed was in 1866, before the creation of the DOJ.
As speculation swirls over whether Acting Attorney General Matt Whitaker might try to rein in special counsel Robert Mueller's investigation, Mueller's team argued to a federal appeals court Monday that Whitaker's appointment "has no effect" on the special counsel's authority and that Mueller continues to have "the full power and independent authority to exercise all investigative and prosecutorial functions of any United States Attorney".
The state of Maryland is suing to have Whitaker removed and be replaced with Rod Rosenstein, who as deputy attorney general is Senate approved and was the second in command to Sessions.More news: Israeli PM Netanyahu resists calls for early election as political crisis worsens
In his first television interview since the midterm elections, President Donald Trump mounted an aggressive defense of his pick for acting attorney general and repeated some of his favorite misleading attacks against the Russian Federation investigation. Many legal experts have disagreed with the department's view.
"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", a Justice Department spokeswoman said. "He's a very smart person, a very respected person", Trump said.
The Justice Department issued a statement Monday defending Whitaker's appointment as "lawful" and said it comports with the Appointments Clause, the Federal Vacancies Reform Act and legal precedent. He's going to do what's right.
Asked by Fox News anchor Chris Wallace whether he would overrule Whitaker if he decides to curtail the Mueller investigation, Trump replied, "I would not get involved".
One of the three senators, Sheldon Whitehouse, accused Trump of installing his "lackey" to undermine the Mueller probe.
They said that his questionable appointment could destabilize any federal court case in the country, and said Rosenstein instead should represent the department.