Since then, Whitaker, who had been Sessions' chief of staff, has faced pressure from Democrats to recuse himself from overseeing special counsel Robert Mueller, based on critical comments he made about the investigation before joining the Justice Department previous year.
Sen. Chuck Grassley (R-Iowa) reiterated what his fellow senator said about Whitaker in his own statement.
He said he was speaking only for himself and he hoped Republicans might join as plaintiffs if a lawsuit goes forward.
However, with Republicans tightening their grip on the Senate, an initially magnanimous-sounding Trump suggested that a divided Congress would bring opportunity for bipartisan cooperation - something in low supply in Washington.
"The president decides who serves in the cabinet".
Organisers say the appointment of Mr Whitaker is a "deliberate attempt to obstruct the special counsel's investigation".
He also said he had not spoken with Whitaker about Mueller's probe.
"With the new attorney general you might be out of a job soon too", Sessions warned, before embracing Mueller around the waist as they both sang the Adele tune together.More news: Recount Likely in Florida Governor Race
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Robert De Niro returned to cameo as Robert Mueller - to which Sessions replied, "You can't arrest me, I quit". "It's illegal. And it means that anything Mr. Whitaker does, or tries to do, in that position is invalid". But the unprecedented nature of his designation, and the issues raised by his background, reinforce the need for him to demonstrate that he will defend the institutional interests of the Department of Justice and the department's independence and non-partisanship-most importantly with respect to his handling of the Mueller investigation. Acting Attorney General Whitaker can take several steps to bolster public confidence.
Schumer also asked Trump for "specific reasons" why he installed Whitaker in the attorney general's office rather than Rosenstein, who is next in the line of succession.
Protesters gathered Thursday in cities from NY to California in a warning to the White House not to derail Robert Mueller's probe into Russian meddling in the 2016 USA presidential election. Whitaker voiced doubts about the scope and direction of the Russian Federation probe and suggested a future attorney general might put the brakes on Mueller by cutting off funding for the investigation.
Speaking to reporters at the White House, Trump defended his choice of Whitaker, saying the former United States attorney for the southern district of Iowa had an excellent reputation and came highly recommended by former Iowa Governor Terry Branstad, who is now Trump's ambassador to China.
Donald Trump emerged from midterm elections promising cooperation only to plunge Washington into turmoil on Thursday (Nov 8) over suspicions that he's trying to kill the Russian Federation collusion probe and an extraordinary intensification of his war with journalists.
I do not know what advice Rosenstein and Wray would give Whitaker.
The incoming House Intelligence Committee chairman is warning new acting attorney general Matt Whitaker any role he plays with the Mueller probe 'will be exposed'. Look for Whitaker to reduce Mueller's operating budget, pull lawyers and investigators now working on Mueller's team back to DOJ, refuse to authorize grand jury subpoenas or indictments, and/or push Mueller to wrap up the probe as soon as possible.