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".
Ethics officials at the Justice Department are likely to review his past work to see if he has any financial or personal conflicts. The recusal left the investigation in the hands of Rosenstein, who appointed Mueller two months later after Trump fired then-FBI Director James Comey.
Trump has continued pressing his view that Mueller leads a team staffed by Democrats operating under an illegal mandate.
Any action by Mr Whitaker could potentially be challenged under the Vacancies Reform Act Trump used to appoint his acting attorney general.
The protest campaign claims on its website that with Sessions ousted and acting Attorney General Matt Whitaker replacing Rod Rosenstein in overseeing the Mueller investigation, Trump "undercut the independence of the investigation" because "Whitaker has publicly outlined strategies to stifle the investigation and can not be allowed to remain in charge of it".
Peskov says Sessions' departure isn't something for the Kremlin to weigh in on.
When President Donald Trump came to the Capital City to support Kris Kobach's bid for governor, he joked with the crowd he kind of hoped the Republican nominee didn't win.
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. Sessions had held on to his job as the nation's top law enforcement officer despite frequent bursts of embarrassingly public criticism from Trump.
In piercing attacks, Trump called Sessions weak and beleaguered, complained that he wasn't more aggressively pursuing allegations of corruption against Democratic rival Hillary Clinton and called it "disgraceful" that Sessions wasn't more serious in scrutinizing the origins of the Russian Federation investigation for possible law enforcement bias - even though the attorney general did ask the Justice Department's inspector general to examine those claims.
Sessions sent the message to Justice Department workers on Thursday, a day after he resigned at Trump's request.
Jeff Sessions imminent departure as attorney general was expected, as he had been repeatedly criticized by Trump for recusing himself from the investigation of potential collusion between the president's campaign and Russian Federation.
In pushing out his attorney-general, the President cast aside one of his earliest and strongest supporters.
Can Whitaker be forced to recuse himself?
Sessions took himself out of decision-making on the probe in March 2017 even after Trump dispatched the White House lawyer to persuade him not to do so, according to a person familiar with the matter.
One reason Sessions endured months of taunting from Trump is that he believed he was protecting the integrity of the Justice Department and was trying to prevent the president from bringing in someone who would politicize it, according to a USA official who has worked with Sessions and frequently talked with him.
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