It's possible he'll still play a role in the Mueller investigation, but the Justice Department made clear Wednesday that Whitaker "is in charge of all matters under the purview of the Department of Justice".
Rosenstein, who appointed Mueller and has overseen his probe from the beginning will stay in his post, according to the official. "Special Counsel Mueller must be allowed to complete his work without interference-regardless of who is AG". Senator Susan Collins and Mitt Romney said it should not be impeded in any way.
King, who sits on the Senate Intelligence Committee, pointed to the passage of a bill supporting Mueller's investigation as the most important action in the wake of the Sessions dismissal, but didn't offer any more specifics about what should be done.
Separately, Justice Department prosecutors in NY secured a guilty plea from Trump's former personal lawyer, Michael Cohen, who said the president directed him to arrange hush money payments before the 2016 election to two women who said they had sex with Trump.
Democrats see this latest move as an attempt to do just that.
By replacing Rod Rosenstein with just-named Acting Attorney General Matt Whitaker as special counsel Robert Mueller's boss on the investigation, Trump has undercut the independence of the investigation.
Never in modern history has a president attacked a Cabinet member as frequently and harshly in public as Trump did Sessions, a former United States senator from Alabama and one of the first members of Congress to back Trump's presidential campaign in 2015.More news: Chrome 71 to remove persistent abusive ads
The mayor, who has gotten into it with the president on Twitter, added: "It is a shocking abuse of power".
The broadsides escalated in recent months, with Trump telling an interviewer that Sessions "never had control" of the Justice Department.
Christie and Trump ran against each other in the 2016 election.
The president asked for Sessions' resignation earlier this week following the midterm elections.
In fact, Whitaker told CNN past year that a future attorney general could limit Mueller by reducing the budget "so low that his investigations grinds to nearly a halt".
But Paul Rosenzweig, a former senior counsel to independent counsel Kenneth Starr, said he thought such a lawsuit would be unlikely to succeed because the US Supreme Court has said the president has wide latitude to fire executive branch officials.
Sessions endured the name-calling in silence, even as he aggressively carried out the administration's conservative policies, such as stripping federal funding from so-called sanctuary cities and states and rescinding protection for young adults brought into the country illegally as children.More news: England consult player dossier on how to defeat New Zealand
Several Republicans expressed concern about Whitaker taking over the Russian Federation probe.
He has also written that he believes the Mueller probe has gone too far and that Trump's finances should be off limits to the special counsel.
"If he doesn't, then Mueller's investigation will eventually start to look like a political fishing expedition".
There has also been a question mark over Mr Rosenstein's future since it was alleged that he had discussed invoking a constitutional clause to oust President Trump.
What does this mean for the Mueller probe?
There had already been hints that Robert Mueller's pre-election "quiet period" was about to come to an end.
"The chances of Mueller being interfered with goes up dramatically" with Whitaker's appointment, he said.More news: France battles to save European Union digital tax proposal