Democrat congressman Jerry Nadler, who is poised to become the leader of the House judiciary committee, said in a sharply-worded statement that Mr Sessions' firing "fit a clear pattern of interference" from the President in the Mueller investigation.
Sessions writes that it was "an incredible honor to work with people who day after day and year after year consistently exceeded expectations".
"In fact, Mr Whitaker told CNN previous year that a future attorney general could limit Mueller by reducing the budget so low that his investigations grinds to nearly a halt". Christie was initially considered as a contender for the attorney general position during the transition.
Congressional Democrats have already called on Whitaker to recuse himself from overseeing the investigation.
Speculation is mounting that Special Counsel Robert Mueller could soon resume issuing subpoenas and indictments, with one report claiming that Donald Trump Jr fears he could be indicted as soon as this week.
Attorney General Jeff Sessions has resigned at the request of President Donald Trump, according to a letter delivered to the White House, after becoming a target of the president's open contempt for ceding control of the Russian Federation investigation.
Whitaker is legally allowed to serve as acting attorney general for no more than 210 days under the Vacancies Reform Act but could be in the post longer while another individual's nomination is pending, according to the Times.
That duty has belonged to Deputy Attorney General Rod Rosenstein, who appointed Mueller and closely monitors his work.
While Whitaker has not stated he believes this is the most prudent or effective outcome of the Russian Federation probe, the legal group Law Works first flagged the 2017 comments in mid-October of this year, when it was announced that Whitaker was tapped to become Sessions' Chief of Staff.
"The president's entitled to an attorney general he has faith in, somebody that's qualified for the job, and I think there will come a time, sooner rather than later, where it will be time to have a new face and a fresh voice at the Department of Justice", Senator Lindsey Graham of SC, who may be in line to head the Judiciary Committee next year, told reporters in August. His spokesman said on Thursday that McConnell has no further comment on the legislation.
Schumer says he finds the timing of Sessions' departure "very suspect".
The Justice Department has declined to answer whether ethics officials have been asked to review if Whitaker should recuse himself from the Mueller probe.
Many voiced concerns Wednesday that Whitaker may have potential conflicts of interest, especially in light of his attacks on the investigation. He was a George W. Bush loyalist - named a former USA attorney in Iowa under Bush - but like many others, he has become a big admirer of Trump along the way. 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.