Attorney General Jeff Sessions’ resignation on Wednesday — and the announcement that his Chief of Staff Matthew Whitaker immediately took over his role overseeing the Department of Justice — rocked Washington and immediately prompted speculation about how the change in leadership could affect Special Counsel Robert Mueller’s investigation.

Trump, after all, had largely blamed Sessions for Mueller's appointment and the special counsel team later probed the AG for hours as part of their investigation into Russian meddling in the 2016 presidential election.

Now Whitaker, 49, leads everything at the Justice Department — including the Russia probe.

READ ATTORNEY GENERAL JEFF SESSIONS' RESIGNATION LETTER

Once a U.S. attorney for the Southern District of Iowa, Whitaker also served as a legal commentator at CNN until he joined the DOJ in September 2017.

During his tenure at the network and roughly a month before he joined the Justice Department, Whitaker penned an op-ed titled “Mueller’s investigation of Trump is going too far,” in which he wrote that Mueller’s delving into the Trump Organization’s finances, separate from the president’s election in 2016, was “deeply concerning to me.”

“It does not take a lawyer or even a former federal prosecutor like myself to conclude that investigating Donald Trump's finances or his family's finances falls completely outside of the realm of his 2016 campaign and allegations that the campaign coordinated with the Russian government or anyone else,” he said. “That goes beyond the scope of the appointment of the special counsel.”

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."

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The acting attorney general thanked Trump Wednesday evening, calling the president's confidence in him "a true honor." He said in a statement he's "committed to leading a fair Department with the highest ethical standards, that upholds the rule of law, and seeks justice for all Americans."

Democrats were quick to sound the alarm on Whitaker's appointment — pointing out his views on the Special Counsel's probe while speculating about the reasoning behind his appointment.

Some Democrats, citing his previous writings and statements, alleged Whitaker would be improperly biased.

Rep. Adam Schiff, D-Calif., acknowledged that Whitaker "has been publicly critical" of Mueller, and alleged that "this may be precisely why the President has chosen to put Whitaker in this role."

"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," House Minority Leader Nancy Pelosi tweeted following Sessions' resignation. "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"

Deputy Attorney General Rod Rosenstein, who has had a contentious relationship with Trump in the past, is currently still overseeing the Russia probe. Rosenstein, who many suspected Trump would fire following reports that he wanted to invoke the 25th Amendment against the president, has a previously scheduled meeting at the White House set for Thursday.

Fox News' Jake Gibson contributed to this report.