Top Democrats, including Senate Minority Leader Chuck Schumer and the ranking members on the House Judiciary and Intelligence Committees, sounded the alarm after Attorney General Jeff Sessions' abrupt resignation on Wednesday, and demanded that Acting Attorney General Matthew Whitaker recuse himself from overseeing the inquiry into Russian meddling in the 2016 elections.

“This is a break-the-glass moment," Sen. Richard Blumenthal, D-Conn., said in a statement.

Sessions' departure, at President Trump's request, came less than two years after he angered Trump by recusing himself from the probe, and any involvement in the Justice Department's Russia investigations, after reports contradicted his claims that he had not met with Russian officials during the 2016 campaign. Sessions had been advised to recuse himself by career ethics officials at the DOJ.

The president tweeted that Whitaker, a former U.S. attorney who currently serves as chief of staff to Sessions, will become the acting attorney general, with a permanent replacement to be announced.

Trump decided not to elevate Deputy Attorney General Rod Rosenstein, who has been grilled by congressional Republicans -- including some who attempted to impeach him -- for his handling of federal investigations related to Trump's team.

In his new capacity, Whitaker, who can hold his post for approximately six months without Senate confirmation, is the head official at the Justice Department. That means, at least for now, that he oversees the Mueller probe and the agency's other federal investigations, including the New York prosecutors' look into the finances of Trump and his former aides.

The DOJ is an Executive Branch department staffed by both career officials, who are shielded from arbitrary termination, and political appointees like Sessions and Whitaker who serve at the president's pleasure.

"This is a break-the-glass moment." — Sen. Richard Blumenthal, D-Conn.

“It is a true honor that the President has confidence in my ability to lead the Department of Justice as Acting Attorney General," Whitaker said in a statement Wednesday evening. "I am committed to leading a fair Department with the highest ethical standards, that upholds the rule of law, and seeks justice for all Americans. I will work with our colleagues in Federal, State, local, and tribal leadership, including our partners in law enforcement and our U.S. Attorneys, to ensure the safety of all Americans and the security of the nation."

He also praised Sessions, saying he was a "dedicated public servant for over 40 years and a "man of integrity who has served this nation well.”

Whitaker has previously written an op-ed in 2017 in which he argued that "any investigation into President Trump's finances or the finances of his family would require Mueller to return to Rod Rosenstein for additional authority under Mueller's appointment as special counsel."

Whitaker continued, "If he were to continue to investigate the financial relationships without a broadened scope in his appointment, then this would raise serious concerns that the special counsel's investigation was a mere witch hunt." (Trump has repeatedly derided the Mueller probe as a "witch hunt," characterizing it as a partisan effort to detract from his accomplishments in office.)

DEMS TO CRACK THE WHIP: HOUSE MAJORITY AFFORDS MAXINE WATERS, ADAM SCHIFF POWER TO INVESTIGATE, SUBPOENA

"It is time for Rosenstein ... to order Mueller to limit the scope of his investigation to the four corners of the order appointing him special counsel," Whitaker concluded. "If he doesn't, then Mueller's investigation will eventually start to look like a political fishing expedition. This would not only be out of character for a respected figure like Mueller, but also could be damaging to the President of the United States and his family -- and by extension, to the country."

And in July 2017, Whitaker told CNN, "I could see a scenario where Jeff Sessions is replaced with a recess appointment, and that attorney general doesn’t fire Bob Mueller, but he just reduces his budget to so low that his investigation grinds to almost a halt.”

At a press conference earlier Wednesday at the White House, Trump emphasized that he could order Justice Department officials to end the Mueller investigation at any point.

"I could fire everybody right now, but I don't want to stop it because politically I don't like stopping it," Trump said. "I am not concerned about anything with the Russian investigation, because it is a hoax. There is no collusion."

Democrats united Wednesday to demand Whitaker's recusal, with some citing his previous writings and statements as supporting the notion that he will be improperly biased. (Republicans made a similar argument, rejected by Democrats, that the political biases expressed by now-fired FBI Agent Peter Strzok, a key player in the Russia and Hillary Clinton probes, should have disqualified him from working on the cases.)

"Acting Attorney General Whitaker must be recused from any involvement from the Russia investigation, and Deputy Attorney General Rosenstein should be allowed to continue to oversee the investigation, unhindered," House Minority Leader Nancy Pelosi, D-Calif., said in a statement Wednesday night.

"Replacing the Attorney General with a non-Senate-confirmed political staffer is highly irregular and unacceptable," Blumenthal, D-Conn., wrote. "Protecting the Special Counsel investigation is more urgent than ever. My Republican colleagues must rise to the challenge and show political backbone by demanding that Mr. Whitaker recuse himself from oversight of the Special Counsel’s investigation."

He added: “I will be introducing legislation to ensure that Congress and the American people see the results of Special Counsel Mueller’s work. I implore my colleagues of both parties to unite behind efforts to ensure that the Special Counsel can continue this work without interference. Any attempt to limit his resources or the scope of his investigation is unacceptable. The world, and history, are watching.”

In another statement, Schumer, D-N.Y., demanded that Whitaker immediately follow Sessions' lead and abandon his role in the Russia probe.

SCHIFF, NADLER, MAXINE WATERS AMONG DEMS LIKELY TO TAKE OVER KEY COMMITEES

“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," Sessions wrote.

And Nadler, who is poised to assume the powerful chairmanship of the House Judiciary Committee when the new Democratic majority is seated in Congress in January, sought a fuller explanation from the White House.

"Americans must have answers immediately as to the reasoning behind @realDonaldTrump removing Jeff Sessions from @TheJusticeDept," Nadler wrote on Twitter. "Why is the President making this change and who has authority over Special Counsel Mueller’s investigation? We will be holding people accountable."

WATCH: SPARKS FLY AT WHITE HOUSE PRESS CONFERENCE, AS TRUMP SPARS WITH CNN REPORTER

California Democratic Rep. Adam Schiff, who is slated to take over the House Intelligence Commitee, echoed Nadler's sentiment, saying that Trump "wants an Attorney General to serve his interest, not the public."

Sources told Fox News that Trump did not call Sessions, but rather White House Chief of Staff John Kelly informed him of the president’s request for him to resign. Sessions is expected to leave the Justice Department by the end of the day, and Whitaker was expected to be sworn in Wednesday.

In his resignation letter, Sessions said was “honored to serve” as attorney general and said his Justice Department “restored and upheld the rule of law – a glorious tradition that each of us has a responsibility to safeguard.”

Sessions' departure from the Justice Department is not unexpected, as the president has signaled changes to his administration after the midterms. But no one faced more rumors of an imminent dismissal than Sessions, and his exit comes less than 24 hours after the conclusion of Tuesday's midterm elections.

Rosenstein headed to the White House for a previously scheduled and unrelated meeting this afternoon.

"I think what ultimately the president is going to start doing is putting pressure on Rod J. Rosenstein, who is in charge of this investigation, is acting attorney general, and really try to get Rod to maybe even cut the budget of Bob Mueller and do something a little more stage crafty than the blunt instrument of firing the attorney general and trying to replace him," Whitaker said in a July interview.

Fox News' Alex Pappas, Jake Gibson, and Chad Pergram contributed to this report.