President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE on Wednesday named Matthew Whitaker as acting attorney general after Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE turned in his resignation from the top Justice Department role, marking a new era of oversight for the DOJ and special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s investigation.

The president’s relationship with his top cop — and Whitaker’s now former boss — had deteriorated over the past two years following Sessions’ decision to recuse himself from the high-profile Russia probe.

In his new capacity, Whitaker will take the reins of overseeing the probe from Deputy Attorney General Rod Rosenstein Rod RosensteinDOJ kept investigators from completing probe of Trump ties to Russia: report Five takeaways from final Senate Intel Russia report FBI officials hid copies of Russia probe documents fearing Trump interference: book MORE, who has been a loyal defender of the investigation since Sessions stepped aside.

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Whitaker is now taking on a much more public role, after serving as Sessions' chief of staff.

Here are five things to know about the new acting attorney general.

Whitaker criticized the Mueller probe

Before joining the Justice Department, Whitaker frequently criticized the very probe he will now be overseeing.

Whitaker has accused the special counsel’s investigation of going "too far," while calling on Rosenstein to "limit the scope" of the investigation.

"The president is absolutely correct. Mueller has come up to a red line in the Russia 2016 election-meddling investigation that he is dangerously close to crossing," Whitaker wrote in an op-ed for CNN in August 2017.

Whitaker also previously wrote an op-ed in The Hill defending the president for firing former FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE, while blasting DOJ officials in the Obama administration for failing to examine scandals at the time when Barack Obama Barack Hussein ObamaMichelle Obama and Jennifer Lopez exchange Ginsburg memories Pence defends Trump's 'obligation' to nominate new Supreme Court justice The militia menace MORE was president.

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Whitaker, however, indicated in an interview with a local Des Moines, Iowa, news station that Trump should not fire Mueller.

“I cannot imagine a scenario where Bob Mueller has done anything inappropriate, or that would demand his termination,” Whitaker said in June 2017.

And already, top Democrats in the House and the Senate have called on Whitaker to recuse himself for comments he’s made about Mueller, including Senate Minority Leader Chuck Schumer Chuck SchumerSenate Democrats introduce legislation to probe politicization of pandemic response Schumer interrupted during live briefing by heckler: 'Stop lying to the people' Jacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee MORE (D-N.Y.) and House Minority Leader Nancy Pelosi Nancy PelosiPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare House lawmakers reach deal to avert shutdown Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill MORE (D-Calif.).

Democrats, who will regain control of the House in the next Congress, have already indicated that they plan to conduct oversight on a series of matters relating to the Trump administration — including his move to oust Sessions.

Whitaker believed there was a ‘strong’ case against Clinton

Whitaker has also expressed a series of opinions about Comey’s decision to not charge Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE for her handling of classified materials during her time as secretary of State.

At the time, Whitaker suggested the FBI was unfair in its treatment of Clinton.

“It’s clear that the FBI was looking for reasons not to bring criminal charges against Hillary Clinton,” Whitaker said in a September 2016 press release for the watchdog group Foundation for Accountability and Civic Trust (FACT), where he was the executive director at the time.

“The critical point the FBI overlooked is that Clinton clearly intended to send and store top secret, classified information on an unsecured, personal server, which I believe is a case that any reasonable prosecutor would bring against her or anyone else who committed such reckless acts,” he continued.

Whitaker, who also expressed respect for Comey, disputed that the FBI chief had the authority to make such a decision instead of then-Attorney General Loretta Lynch.

He, however, is not alone in these opinions -- many other Republicans have criticized Comey's decision not to charge Clinton.

House Republicans have been investigating the decision-making of top FBI and DOJ officials during the 2016 presidential race -- an investigation that may be coming to a close now that the House has flipped to Democratic control.

Whitaker defended Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE for taking the Trump Tower meeting

Whitaker defended the president’s eldest son for meeting with a Kremlin-linked lawyer who promised to give dirt on Clinton’s campaign amid the heated presidential race.

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Mueller is believed to be scrutinizing circumstances of the June 2016 Trump Tower meeting. Despite repeatedly changing the narrative of this meeting, Trump Jr. has denied receiving any incriminating information.

"You would always take that meeting,” Whitaker said during an appearance on CNN in July 2017 that recently re-surfaced.

“If you have somebody that you trust that is saying that you need to meet with this individual because they have information about your opponent, you would take that meeting,” he added.

Despite his defense, in a August 2017 op-ed in The Hill, Whitaker argued that the matter merits “serious investigation” as he pointed out another case of a Democratic official reportedly meeting with Ukrainian officials.

Citing reports, Whitaker said news that a Democratic National Committee (DNC) official met with Ukrainian officials in an effort to investigate ties Trump and his former campaign manager Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE had to Russia merits investigation, but it is being "swept under the rug."

“Given what we know today about both situations, it’s clear they both merit serious investigation,” Whitaker wrote in the op-ed. “Foreign influence in our politics is nothing new but it is very concerning and should be investigated. The DNC/Ukraine connection is serious, and the public deserves answers.”

His remarks echoed those of Trump at the time, who opined that the matter wasn't receiving its due attention.

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In another potential Mueller tie, Whitaker is friends with Sam Clovis

Sam Clovis, who previously served as the Trump campaign's co-chairman, tapped Whitaker to serve as his chairman during a bid to be Iowa’s state treasurer in 2014.

Whitaker, who was then a managing partner of Whitaker Hagenow & Gustoff LLP, a Des Moines-based law firm, praised Clovis at the time for being an experienced public servant, according to reports.

“When the opportunity came for me to have my name placed on the ballot as the Republican nominee for Treasurer of State, Matt became the logical person to chair my campaign committee,” Clovis reportedly said in a statement at the time.

Clovis, however, has also become entangled in Mueller’s probe, prompting some critics to claim these ties create a conflict of interest for Whitaker to oversee the Mueller probe.

The Washington Post reported earlier this year that Clovis was the campaign official who encouraged George PapadopoulosGeorge Demetrios PapadopoulosTale of two FBI cases: Clinton got warned, Trump got investigated Trump says he would consider pardons for those implicated in Mueller investigation New FBI document confirms the Trump campaign was investigated without justification MORE to set up an “off the record” meeting with Russian officials.

Papadopoulos, who pleaded guilty to charges that he lied to federal investigators about his contacts with Russians during the campaign, has been sentenced to two weeks in prison.

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Clovis, while acknowledging he remains friends with Whitaker, disputed that those ties impact Whitaker's ability to oversee the investigation.

“It’s not relevant and Matt has high integrity. I’m very happy for him and he’ll do a fantastic job,” Clovis told the Post on Wednesday.

Whitaker is considered a Trump loyalist

Trump is reportedly fond of Whitaker, a former football player who frequently went back and forth between the West Wing and the DOJ amid the president’s strained relations with Sessions.

“[Whitaker] has frequently visited the Oval Office and is said to have an easy chemistry with Mr. Trump,” The New York Times reported in September amid premature reports that Rosenstein had resigned.

Whitaker's name first appeared as a possible replacement after the Times published a bombshell report that said Rosenstein had spoken to other officials about wearing a wire to record Trump after he fired Comey last May. The story also said Rosenstein had discussed the possibility of Cabinet officials invoking the 25th Amendment to remove Trump from office last year.

Rosenstein and the DOJ have fiercely disputed the Times report, claiming the comments were made in jest. Despite their denials, the report sparked nearly instant speculation that Rosenstein could be on his way out the door.

While questions about Rosenstein's fate bubbled up, Trump soon dispelled claims he would fire the No. 2 official.

Following that episode, however, Trump made sure to call and reassure Whitaker that he had faith in him, according to the Times.

In addition to his public defense of the president, Whitaker has also followed Sessions’ lead on having the agency align with the president’s key priorities, including immigration and violent crime, a current DOJ official told the Times in September.

While the president has said the appointment of Whitaker is temporary, it could be a test-run for other future roles in his administration.