Republicans warned Trump a year ago against replacing Sessions, but some of that resistance appeared to break down by late August, when several key GOP senators signaled to Trump that he could find a new attorney general after the midterm elections.
The timing for when a final report would be submitted by the special counsel's office is still unclear, and sources say there is also no clear timeline for when Mueller will wrap his investigation.
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.
Trump's relentless attacks on Sessions came even though the Alabama Republican was the first USA senator to endorse Trump and despite the fact his crime-fighting agenda and priorities, particularly his hawkish immigration enforcement policies, largely mirrored the president's.
That drew criticism from Democrats in Congress because Whitaker, who would now take over responsibility for overseeing Mueller and his investigation, has been critical of the Mueller probe, saying it should be scaled back.
The resignation, in a one-page letter to Trump, came one day after Republicans lost control of the House and was the first of several expected post-midterms Cabinet and White House departures.
Schumer, in a separate press conference with reporters, warned that attempts by Sessions's successor or Trump to interfere with the investigation would spark a "constitutional crisis". "There is no mistaking what this means, and what is at stake: this is a constitutionally perilous moment for our country and for the president".
The swift firing may have also been a way for Mr Trump to get ahead of any efforts from the newly-empowered Democrats to reopen the House intelligence committee's investigation into ties between Mr Trump's campaign team and Russian Federation.
"It is time for Rosenstein, who is the acting attorney general for the purposes of this investigation, to order Mueller to limit the scope of his investigation to the four corners of the order appointing him special counsel". Whitaker did, however, believe that Sessions had no choice but to recuse himself from the matter, the person said. Four former Trump campaign officials - including his onetime national security adviser Michael Flynn and campaign chairman Paul Manafort - are among those who have pleaded guilty.
In addition to Sessions' ouster, Democrats are also interested in looking into Mr. Trump's tax returns, and global business dealings.
He added that if the firing of Mr Sessions is not immediately investigated, the Democratic Congress would make it a priority when it is sworn in in January.
Sessions took the lead in imposing Trump's travel ban targeting a number of mostly Muslim nations, canceling the so-called Dreamer program that let the children of undocumented immigrants remain in the US and forcing the separation of migrant families as part of a border crossing crackdown.