WASHINGTON (CN) – Attorney General Jeff Sessions resigned Wednesday as the country’s chief law enforcement officer, saying in his resignation letter to President Donald Trump that he was doing so at “your request.”

“I came to work at the Department of Justice every day determined to do my duty and serve my country,” Sessions wrote. “I have done so to the best of my ability, working to support the fundamental legal processes that are the foundation of justice.”

Trump announced in a separate tweet that he was naming Sessions’ chief of staff Matthew Whitaker, a former United States attorney from Iowa, as acting attorney general.

The Justice Department then announced that Whitaker is taking over supervision of Special Counsel Robert Mueller’s Russia investigation from Rod Rosenstein, the embattled deputy attorney general.

“The Acting Attorney General is in charge of all matters under the purview of the Department of Justice,” DOJ spokeswoman Sarah Isgur Flores said when asked who would be supervising Mueller’s probe in the wake of Attorney General Jeff Sessions’ resignation.

Rosenstein had been supervising the Russia probe – overseeing Mueller’s budget, indictments and subpoenas – since it began in May 2017 because of Sessions’ recusal from all matters related to the 2016 presidential campaign.

Whitaker appeared on a CNN panel last year and floated the idea of a potential Sessions replacement taking an ax to the special counsel investigation’s budget instead of outright firing Mueller.

“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 so low that his investigation grinds to almost a halt,” Whitaker said.

Response to these developments was swift and decidedly negative.

Trevor Potter, president of the Campaign Legal Center, said in a statement that Session’s forced resignation “is a dangerous and authoritarian step” that and “threatens the rule of law.”

“The President is seeking to control or influence an investigation into activities of his presidential campaign and some of its top officials – a number of whom have already been charged and convicted of criminal activities,” he continued.

“The Special Counsel was appointed because of the President’s repeated attempts to interfere with the FBI and DOJ-led investigation. It is important that the DOJ’s law enforcement process remain independent of the political process. It is just as important as ever that Special Counsel Mueller be able to do his job and complete a thorough, independent investigation free from interference from the new pick for Attorney General or anyone else that reports to the President,” Potter said.

Over the course of Sessions’ tenure, Trump has repeatedly criticized the Justice Department  and the FBI in tweets and in comments made to the media.

During one “Fox & Friends” interview before he was fired, Trump said of Sessions: “he never took control of the Justice Department.”

The president also commented on the attorney general’s management of the department saying “it’s sort of an incredible thing.”

Last summer, Sessions felt moved to respond, issuing a statement in which he said the department wouldn’t become “improperly influenced by political considerations.”

He then added that under his leadership the department had “unprecedented success at effectuating the president’s agenda, one that protects the safety and security and rights of the American people, reduces crime and enforces our immigration laws, promotes economic growth and advances liberty.”

The mounting tension between Sessions and Trump was inflamed further by a series of dual losses for former Trump-connected officials in the courts.

The president’s former campaign chairman Paul Manafort was convicted on eight felony charges on August 21 at a federal court in Virginia. The president’s onetime personal attorney, Michael Cohen, pleaded guilty to campaign finance violations the same day.

Cohen told prosecutors he attempted to influence a federal election by using campaign contributions as hush money.

The contributions of $150,000 and $130,000 line up with what former Playboy model Karen McDougal and porn star Stormy Daniels said they received to remain quiet about their respective affairs with Trump.

Like Sessions , Cohen too was criticized by Trump on Twitter: the president accused Cohen of “making up stories” in order to “get a great deal” from prosecutors after Cohen entered his plea.

Before being appointed as attorney general, Sessions established himself as a loyalist to Trump: he was the first U.S. senator to endorse him for president.

Sessions relationship with the president turned rocky not long after his appointment when the then newly-minted attorney general recused himself from the Russia probe, drawing the president’s ire.

Over a year of Trump’s grievances with Sessions are documented on Twitter and in various media programs.

When Sessions first recused himself, Trump said to hosts of “Fox & Friends:” “What kind of a man is this?”

“You know, the only reason I gave him the job was because I felt loyalty, he was an original supporter,” Trump said.

The president also lashed out at his former attorney general this past June, his outrage over the Russia probe directed at Sessions in a tweet:

“The Russian Witch Hunt Hoax continues, all because Jeff Sessions didn’t tell me he was going to recuse himself … I would have quickly picked someone else,” Trump wrote on June 5. “So much time and money wasted, so many lives ruined…and Sessions knew better than most that there was No Collusion!” [Emphasis original.]

A year before, Trump berated Sessions, calling him “beleaguered” and “disgraceful” at one point on television and on Twitter, questioned his loyalty.

“Attorney General Jeff Sessions has taken a VERY weak position on Hillary Clinton crimes (where are E-mails & DNC server) & intel leakers,” Trump wrote last July.