WASHINGTON

U.S. Attorney General Jeff Sessions was fired on Wednesday after receiving unrelenting criticism from President Trump for recusing himself from an investigation into Russia’s role in the 2016 presidential race.

In a step that could have implications for the investigation led by Special Counsel Robert Mueller, President Trump replaced Mr. Sessions with Matthew Whitaker, who will be acting attorney general. He had been Mr. Sessions’ chief of staff.

The top Democrat in the U.S. Senate immediately called on Mr. Whitaker to recuse himself from the Mueller probe.

“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,” Senate Democratic leader Chuck Schumer said in a statement.

In an opinion piece for CNN that appeared on Aug. 6, 2017, while he was a commentator for the network, Mr. Whitaker, a former U.S. attorney, said Mr. Mueller would be crossing a line if he investigated the Trump family’s finances. The piece was titled: “Mueller’s investigation of Trump is going too far.”

President Trump’s personal lawyer Rudy Giuliani told Reuters on Tuesday that he assumed Mr. Sessions’ departure was “not going to affect” the Mueller investigation.

Deputy Attorney General Rod Rosenstein, who is supervising the Russia investigation and has also faced criticism from the president, was seen by Reuters entering the White House on Wednesday afternoon.

A spokesman for the special counsel’s office declined to comment on Mr. Sessions’ resignation and what it means for the probe.

President Trump announced Mr. Sessions’ departure on Twitter and thanked him for his service. Mr. Sessions said in a letter to President Trump that he had resigned at the president’s request.

Mr. Sessions’ exit had been widely expected to come soon after Tuesday’s congressional elections, in which Republicans retained their majority in the U.S. Senate but lost control of the U.S. House of Representatives.

Never in modern history has a president attacked a cabinet member as frequently and harshly in public as President Trump did Mr. Sessions, 71, who had been one of the first members of Congress to back his presidential campaign in 2015.

Democratic Rep. Jerrold Nadler of New York, who is expected to chair the House Judiciary Committee starting in January, demanded answers in a tweet about President Trump’s reasons for the firing.

“Why is the President making this change and who has authority over Special Counsel Mueller’s investigation? We will be holding people accountable,” Congressman Nadler said on Twitter.

Mr. Mueller’s probe, operating under the auspices of the U.S. Justice Department, already has yielded criminal charges against several Trump associates and has clouded the Trump presidency for many months.

Republicans repeatedly had urged President Trump not to oust Mr. Sessions, a former conservative U.S. senator from Alabama, before the elections lest it create political fallout. They also had argued that Mr. Sessions should be allowed a graceful exit after he doggedly carried out President Trump’s agenda on illegal immigration and other administration priorities.

President Trump was only a few weeks into his presidency in March 2017 when Mr. Sessions upset him. Rejecting White House entreaties not to do so, Mr. Sessions stepped aside from overseeing the FBI’s probe of potential collusion between the Trump presidential campaign and Moscow. Mr. Sessions cited news reports of previously undisclosed meetings he had with Russia’s ambassador to Washington as his reason for recusal.

Mr. Rosenstein then took over supervision of the Russia investigation and appointed Mr. Mueller in May 2017 as the Justice Department’s special counsel to take over the FBI’s Russia probe after President Trump fired FBI Director James Comey.

A permanent replacement for Mr. Sessions must be confirmed by the U.S. Senate, which Republicans will continue to control as a result of Tuesday’s midterm elections.

Mr. Mueller is pursuing an investigation into whether President Trump’s campaign colluded with Russia, whether the president unlawfully tried to obstruct the probe and possible financial misconduct by President Trump’s family and associates. Mr. Mueller has brought charges against President Trump’s former campaign chairman and other campaign figures, as well as against 25 Russians and three firms accused of meddling in the campaign to help President Trump win.

President Trump has denied his campaign colluded with Russia.

President Trump publicly seethed over Mr. Sessions’ recusal and said he regretted appointing him. On Twitter, he blasted Mr. Sessions as “VERY weak” and urged him to stop the Russia investigation. In July 2017, he told the New York Times that if he had known Mr. Sessions would recuse himself, he never would have appointed him attorney general.

Washington Post journalist Bob Woodward wrote in his book “Fear” that President Trump, talking to a White House secretary, disparaged Mr. Sessions as “mentally retarded” and a “dumb Southerner” while mocking his accent.

There also were news reports in the weeks after Mr. Mueller’s appointment that Mr. Sessions had offered to resign. Mr. Sessions usually remained quiet on President Trump’s criticism, but defended himself in February 2018 after a Trump tweet criticizing his job performance by saying he would perform his duties “with integrity and honor.”

In August, Mr. Sessions punched back harder after President Trump said in a Fox News interview that Mr. Sessions “never took control of the Justice Department.” Mr. Sessions issued a statement saying he “took control of the Department of Justice the day I was sworn in” and vowed not to allow it to be “improperly influenced by political considerations.”

As for his own involvement with Russia, Mr. Sessions was questioned in January by Mr. Mueller’s team and has offered shifting public accounts. He has said nothing improper transpired in his meetings during the campaign with Russian Ambassador Sergei Kislyak. In congressional testimony in November, he said he now recalled a meeting during the 2016 campaign in which a campaign adviser, with President Trump present, offered to use connections with Moscow to arrange a meeting between President Trump and Russian President Vladimir Putin.