The appointment generated immediate controversy given Mr Whitaker's previous public comments criticising the probe. He did not mention that White House chief of staff John Kelly had called Sessions beforehand to ask for his resignation.
Critics worry Whitaker may be unlikely or unwilling to defend the investigation, given his history of partisanship and loyalty to Trump.
She went on to argue that, "given his record of threats to undermine and weaken the Russian Federation investigation", Mr Whitaker should follow in Mr Sessions' footsteps and recuse himself.
Sessions, a Republican and former Alabama senator, has been the focus of public attacks by Trump, who criticized him for recusing himself from the Russian Federation investigation because of his role on the Trump campaign team before becoming attorney general.
The deputy attorney general appointed Mr Mueller to lead the inquiry after Mr Trump fired Federal Bureau of Investigation director James Comey in 2017.
"Essentially we need to shame him into acting in compliance with the law", she said.More news: Women candidates make history in U.S. 2018 midterm elections
Special counsel Robert Mueller and his team of prosecutors are in the process of writing a final report on the almost 18-month long investigation into Russian meddling during the 2016 election, sources tell ABC News. Nadler and other Democrats on the committee suggested the shakeup appears to be an effort to alter oversight of special counsel Robert Mueller's probe in a way that favors the president.
Good cause is a hard thing to show, especially since Mueller's actions were approved by the deputy attorney general, said Jens Ohlin, a professor of criminal law at Cornell Law School.
And I think that's what's critical.
Graham said in a statement: 'I talked with the President about a permanent replacement for Attorney General.
Could a Mueller firing be challenged on legal grounds? A court might then issue a temporary restraining order blocking his removal.
"I'm disappointed in the attorney general for many reasons", Trump told reporters in September.More news: I will do things differently as Prince of Wales, says William
In July 2017 Mr Trump told the New York Times: "Sessions should have never recused himself, and if he was going to recuse himself, he should have told me before he took the job and I would have picked somebody else".
The possibility that Sessions was sacked could open Whitaker's appointment to court challenge by a range of people, including Mueller, impacted by his actions.
He said he was "not concerned about anything with the Russian investigation, because it is a hoax". Cox was eventually fired by the U.S. solicitor general.
How could Whitaker impede Mueller without firing him? They also want Congress to take steps to protect the special counsel's office.
Mr Whitaker would be required to notify Congress of such a decision.
The president deflected questions about Sessions' expected departure at a White House news conference Wednesday.More news: England's George Burgess cops four-match ban for eye-gouging Kiwis captain
Mr Whitaker has publicly criticised Mr Mueller's conduct several times since the inquiry began in May a year ago and taken the side of President Trump, who has frequently... 'It's illegal. And it means that anything Mr. Whitaker does, or tries to do, in that position is invalid.