Washington (CNN) Democratic congressional leaders reacted to the news of Attorney General Jeff Sessions' firing on Wednesday by warning that special counsel Robert Mueller's investigation into potential Russia collusion must be safeguarded from interference.

They also called for the new acting attorney general, who has been openly critical of Mueller and the investigation, to recuse himself from oversight of the inquiry.

House Democratic leader Nancy Pelosi of California said on Twitter , "It is impossible to read Attorney General Sessions' firing as anything other than another blatant attempt" by President Donald Trump "to undermine" and "end" the Mueller probe.

Senate Democratic leader Chuck Schumer of New York said at a news conference in Washington that "protecting Mueller and his investigation is paramount" and it "would create a constitutional crisis if this were a prelude to ending or greatly limiting the Mueller investigation."

Both called for the recusal of Matthew Whitaker, who Trump announced on Twitter will become the new acting attorney general.

Whitaker, who was Sessions' chief of staff, is expected to take charge of the Russia investigation from Deputy Attorney General Rod Rosenstein. Whitaker argued in a CNN op-ed last year that Mueller was "dangerously close to crossing" a red line following reports that the special counsel could be examining financial records tied to the Trump Organization. In the same op-ed, Whitaker called for the investigation to be limited in its scope.

"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," he wrote. "If he doesn't, then Mueller's investigation will eventually start to look like a political fishing expedition."

In a statement released after his news conference Wednesday, Schumer also called on Whitaker to recuse himself from oversight of the investigation.

"Given his previous comments advocating defunding and imposing limitations on the Mueller investigation, Mr. Whitaker should recuse himself from its oversight for the duration of his time as acting attorney general," Schumer said in the statement.

Pelosi made a similar statement on Twitter , saying, "Given his record of threats to undermine & weaken the Russia investigation, Matthew Whitaker should recuse himself from any involvement in Mueller's investigation. Congress must take immediate action to protect the rule of law and integrity of the investigation. #FollowTheFacts."

Sessions resigned his position as attorney general in a letter to the President, writing, "At your request, I am submitting my resignation."

Mueller is investigating potential links or coordination between Trump's 2016 campaign associates and the Russian government. The President has consistently denied that there was any collusion and has attacked the inquiry as a "witch hunt."

A senior GOP congressional aide reaffirmed the previous position for GOP leadership that Trump should not fire Mueller.

"He would be playing with fire," the aide said.

Democratic Rep. Jerry Nadler of New York, who is poised to become the chairman of the House Judiciary Committee when Democrats take control of Congress, said in a statement that "The firing of Attorney General Jeff Sessions fits a clear pattern of interference from President Donald Trump in the work of the Department of Justice and the ongoing criminal investigation being conducted by Special Counsel Robert Mueller."

He added, "We are immediately issuing multiple letters to key officials demanding that they preserve all relevant documents related to this action to make sure that the investigation and any evidence remains safe from improper interference or destruction."

Democratic Rep. Adam Schiff of California, who is expected to take over the gavel on the House Intelligence Committee, vowed that Democrats will "protect the rule of law."

"President Trump just removed Jeff Sessions. He wants an Attorney General to serve his interest, not the public," Schiff tweeted . "Mueller's investigation and the independence of the DOJ must be protected. Whitaker and any nominee must commit to doing both. We will protect the rule of law."

Schumer said at his news conference that he hopes "if there is a constitutional crisis that our Republican colleagues would join us in thwarting the President from creating that crisis."

The Senate Democratic leader said he finds the timing of Sessions' departure "very suspect." He went on to say, "Our paramount view is that any attorney general, whether this one or another one, should not be able to interfere with the Mueller investigation in any way. They should not be able to end it. They should not be able to limit it. They should not be able to interfere with Mueller going forward and doing what he thinks is the right thing."

This story has been updated.