WASHINGTON – Attorney General Jeff Sessions' departure from the Justice Department immediately moves oversight of the ongoing Russia investigation to his interim successor, Matthew Whitaker, who has publicly advocated for a limited inquiry.

Sessions resigned Wednesday at President Donald Trump's request.

The move that prompted Democrats to warn that the president trying to derail the investigation by Russia special counsel Robert Mueller. House Democratic Leader Nancy Pelosi said Trump's firing of Sessions was a "blatant attempt" to undermine Mueller's work.

Whitaker, in a 2017 article published by CNN before joining the Justice Department as Sessions' chief of staff, called on Deputy Attorney General Rod Rosenstein to "limit the scope" of the Mueller investigation and block review of President Donald Trump's family finances.

Until Wednesday, Rosenstein had been managing the inquiry following Sessions' recusal from the matter last year. Rosenstein was at the White House Wednesday for what administration officials described as a previously scheduled meeting.

Trump has publicly feuded with Sessions over the Mueller probe and has at times also extended his attacks to Rosenstein for what he calls the Russian "witch hunt."

In 2017, Whitaker wrote: "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."

"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."

Whitaker's installation quickly pushed Democras to call for Mueller's inquiry to be shielded from any undue interference.

Senate Democratic Leader Chuck Schumer, D-N.Y., vowed to protect Mueller, saying it would create a constitutional crisis if Session’s departure were a prelude to ending, or greatly limiting, Mueller’s investigation.

“And I hope President Trump, and those he listens to, will refrain from that,” Schumer said as he found out about Session’s resignation in the middle of a news conference he was giving on Capitol Hill. “I find the timing very suspect.”

Asked if Democrats can do anything to prevent a successor they don’t like, Schumer said he hopes Republican lawmakers would join with Democrats if necessary to thwart Trump from creating a crisis.

"It is impossible to read Attorney General Sessions’ firing as anything other than another blatant attempt by @realDonaldTrump to undermine & end Special Counsel Mueller’s investigation," Pelosi wrote on Twitter Wednesday.

New York Rep. Jerrold Nadler, the ranking Democrat on the House Judiciary Committee, said the president's action represented a "perilous moment" for the country.

"Donald Trump may think he has the power to hire and fire whomever he pleases, but he cannot take such action if it is determined that it is for the purposes of subverting the rule of law and obstructing justice," Nadler tweeted.

Meanwhile, California Sen. Dianne Feinstein, the ranking Democrat on the Senate Judiciary Committee, called for Whitaker to appear before the Senate to pledge that he would not impede Mueller's inquiry.

“Rod Rosenstein has been capably overseeing Robert Mueller’s work and should continue in that role," Feinstein said Wednesday. "No one who lacks Senate confirmation should be placed in charge of this investigation, especially Matthew Whitaker who publicly criticized Robert Mueller’s work just last year."

Back in 2016, Whitaker wrote that he believed Hillary Clinton should have been indicted over her use of a private email server.

In an opinion piece for USA TODAY, Whitaker disagreed with then-FBI Director James Comey that "no reasonable prosecutor" would indict Clinton for handling classified documents as secretary of State on a private email server.

"I believe myself to have been a reasonable prosecutor, and when the facts and evidence show a criminal violation has been committed, the individuals involved should not dictate whether the case is prosecuted," wrote Whitaker, a former U.S. attorney.

He penned the piece while serving as executive director of the Foundation for Accountability and Civic Trust, a conservative watchdog group.

The move against Sessions, while expected, also rankled some Republicans who expressed concern about the change in oversight of the Russia probe.

"Special counsel Mueller must be allowed to complete his work without interference -- regardless who is AG," said Sen. Susan Collins, R-Maine.