Attorney General Jeff Sessions resigned at the request of President Donald Trump on Wednesday, a move that potentially threatens the independence of special counsel Robert Mueller’s investigation into the president.

“At your request, I am submitting my resignation,” Sessions wrote in an undated letter released Wednesday afternoon. A senior White House official and two sources close to Trump characterized Sessions’ exit as an explicit “firing.” It came hours after Trump declared victory in Tuesday’s midterm elections that saw Republicans increase their majority in the Senate.

Trump announced that Matthew G. Whitaker, Sessions’ chief of staff and a Mueller critic, would become acting attorney general with a permanent replacement to be nominated “at a later date.” That person must be confirmed by the Senate.

The Justice Department indicated Whitaker will assume oversight of Mueller and his office from Deputy Attorney General Rod Rosenstein.

“The Acting Attorney General is in charge of all matters under the purview of the Department of Justice,” a spokesperson told The Daily Beast. (Mueller’s office declined to comment.)

In an op-ed last year, Whitaker called on Mueller “limit the scope of his investigation” into the president. He wrote that Mueller was “dangerously close to crossing” Trump’s self-declared “red line” of investigating Trump family finances.

For the time being at least, Trump’s legal team says that Sessions’s firing hasn’t altered their timeline for the president huddling with his lawyers to craft answers to Mueller's written questions.

“We will move ahead same as we always intended,” Trump attorney Rudy Giuliani told The Daily Beast on Wednesday night. “[The president] will be away until mid-next week and that’s when we can address it.”

Giuliani had previously said that things like midterm election campaigning and a recent wave of attempted bombings and an attack on a Pittsburgh synagogue last month consumed too much of Trump’s time, making it hard for he and his lawyers to schedule a meeting.

Democrats on Capitol Hill were alarmed by the apparent move against Mueller. Sen. Chuck Schumer (D-NY) found out about Sessions’ mandatory resignation as he was briefing reporters on Wednesday afternoon.

“Oh!” he said, as an aide slid a piece of paper on to his podium. “I just heard the news but I’d say this, protecting Mueller and his investigation is paramount. It would create a constitutional crisis if this were a prelude to ending or greatly limiting the Mueller investigation, and I hope President Trump and those he listens to will refrain from that.”

Sen. Mark Warner (D-VA), vice chairman of the Senate Select Committee on Intelligence investigating Trump and Russia, echoed Schumer.

“No one is above the law and any effort to interfere with the Special Counsel’s investigation would be a gross abuse of power by the president,” said. “While the president may have the authority to replace the attorney general, this must not be the first step in an attempt to impede, obstruct or end the Mueller investigation.”

Others Democratic leaders were bullish.

Rep. Jerry Nadler (D-NY), who is in line to chair the House Judiciary Committee, tweeted that “Americans must have answers immediately as to the reasoning” behind Sessions’ removal.

As chairman, Nadler will have the power to attempt to get those answers through subpoenas once he takes the gavel in January when a new Congress is seated.

Rep. Adam Schiff (D-CA), the top Democrat on the House intelligence committee who will also get subpoena power come January, tweeted Trump “wants an Attorney General to serve his interest, not the public. Mueller’s investigation and the independence of the DOJ must be protected. Whitaker and any nominee must commit to doing both. We will protect the rule of law.”

Soon after Sessions’ exit was announced, yet more drama unfolded when Deputy Attorney General Rod Rosenstein was seen entering the White House for a meeting. The “previously scheduled” meeting led to rampant speculation that the deputy AG, who has also faced Trump’s public wrath and oversees the Mueller probe, could resign in the wake of Sessions’ ouster. However, after an hour, Rosenstein was seen leaving the White House without any such resignation.

From Friends to Enemies

The fall of Sessions comes nearly three years after he endorsed Trump for president, the first Republican senator to do so. In 2017, Trump rewarded Sessions’ loyalty by nominating him to be attorney general where he pursued a rollback of Obama-era criminal-justice reform and an aggressive policy of prosecuting immigrants who entered the U.S. illegally.

But Sessions recused himself from overseeing the Justice Department’s investigation related to Trump and Russia after it was revealed that he had failed during his confirmation hearings to disclose meetings with Russian officials as a Trump campaign advisor in 2016.

The recusal opened Sessions up to an unending stream of yearlong criticism from Trump, whose extreme emphasis on personal loyalty has often smudged the line dividing employees from public servants.

Trump has said that once Sessions is fired or departed from his administration, he would be happy to never see him “ever again,” a source told The Daily Beast.

The president has over the past year repeatedly mused to lawyers, confidants, and West Wing aides about his recurring desire to sack his attorney general, who Trump felt had failed to protect him from the Russia probe. Trump was also angry at Sessions for declining to appoint a special counsel to investigate the FBI, Trump’s bête noire, and failing to investigate Hillary Clinton’s emails to Trump’s satisfaction.

For his part, Sessions was “pissed” about Trump’s public humiliation of him, Sessions’ close allies previously told The Daily Beast. Trump has lashed out numerous times at Sessions in public and in private, including repeatedly deriding Sessions as a “retard” or mentally “retarded,” according to sources with direct knowledge of the president’s comments.

With Sessions recused, Rosenstein appointed Mueller to take over the Russia investigation following Trump’s firing of FBI Director James Comey in May 2017. The probe has expanded to explore any links between the Trump campaign and the Kremlin—and, reportedly, whether Trump’s repeated past attempts to fire Sessions amount to obstruction of justice.

Last month, Rosenstein defended Mueller’s investigation as “appropriate and independent,” which stands in contrast to the president’s metronomic insistence that the special counsel probe is a “witch hunt.”

“People are entitled to be frustrated, I can accept that,” Rosenstein told the Wall Street Journal on October 17. “But at the end of the day, the public will have confidence that the cases we brought were warranted by the evidence, and that it was an appropriate use of resources.”

For months, White House attorneys warned Trump that Sessions’ ouster would simply add fuel to the Mueller investigation’s pursuit of obstruction charges—a calculation that Trump has apparently dismissed.