WASHINGTON (AP) - Attorney General Jeff Sessions was pushed out Wednesday after enduring more than a year of blistering and personal attacks from President Donald Trump, who inserted in his place a Republican Party loyalist with authority to oversee the remainder of the special counsel's Russia investigation.

The move has potentially ominous implications for special counsel Robert Mueller's probe given that the new acting attorney general, Matthew Whitaker, until now Sessions' chief of staff, has questioned the inquiry's scope and spoke publicly before joining the Justice Department about ways an attorney general could theoretically stymie the investigation.

Congressional Democrats, concerned about protecting Mueller, called on Whitaker to recuse himself from overseeing the investigation in its final but potentially explosive stages.

That duty has belonged to Deputy Attorney General Rod Rosenstein, who appointed Mueller and closely monitors his work.

The resignation, in a one-page letter to Trump, came one day after Republicans lost control of the House of Representatives and was the first of several expected post-midterms Cabinet and White House departures. Though Sessions was an early and prominent campaign backer of Trump, his departure letter lacked effusive praise for the president and made clear the resignation came "at your request."

"Since the day I was honored to be sworn in as attorney general of the United States, I came to work at the Department of Justice every day determined to do my duty and serve my country," Sessions wrote.

The departure was the culmination of a toxic relationship that frayed just weeks into Sessions' tenure, when he stepped aside from the Russia investigation because of his campaign advocacy and following the revelation that he had met twice in 2016 with the Russian ambassador to the U.S.

Trump blamed the recusal for the appointment of Mueller, who took over the Russia investigation two months later and began examining whether Trump's hectoring of Sessions was part of a broader effort to obstruct the probe.

The investigation has so far produced 32 criminal charges and guilty pleas from four former Trump aides. But the work is not done and critical decisions await that could shape the remainder of Trump's presidency.

Mueller's grand jury, for instance, has heard testimony for months about Trump confidant Roger Stone and what advance knowledge he may have had about Russian hacking of Democratic emails. Mueller's team has also been pressing for an interview with Trump. And the department is expected at some point to receive a confidential report of Mueller's findings, though it's unclear how much will be public.

Separately, Justice Department prosecutors in New York secured a guilty plea from Trump's former personal lawyer, Michael Cohen, who said the president directed him to arrange hush-money payments before the 2016 election to two women who said they had sex with Trump.

Trump had repeatedly been talked out of firing Sessions until after the midterms, but he told confidants in recent weeks that he wanted Sessions out as soon as possible after the elections, according to a Republican close to the White House who was not authorized to publicly discuss private conversations.

The president deflected questions about Sessions' expected departure at a White House news conference Wednesday. He did not mention that White House chief of staff John Kelly had called Sessions beforehand to ask for his resignation. The undated letter was then sent to the White House.

The Justice Department did not directly answer whether Whitaker would assume control of Mueller's investigation, with spokeswoman Sarah Isgur Flores saying he would be "in charge of all matters under the purview of the Department of Justice."

Rosenstein remains at the department and could still be involved in oversight. He has previously said that he saw no basis for firing Mueller. Trump said Wednesday that he did not plan to stop the investigation.

Without Sessions' campaign or Russia entanglements, there's no legal reason Whitaker couldn't immediately oversee the probe. And since Sessions technically resigned instead of forcing the White House to fire him, he opened the door under federal law to allowing the president to choose his successor instead of simply elevating Rosenstein, said University of Texas law professor Stephen Vladeck.

"Sessions did not do the thing he could have done to better protect Rosenstein, and through Rosenstein, the Mueller investigation," Vladeck said.

That left Whitaker in charge, at least for now, though Democrats, including Rep. Nancy Pelosi and Sen. Chuck Schumer, said he should recuse himself because of his comments on the probe. Rep. Jerry Nadler, the top Democrat on the House Judiciary Committee, said he wants "answers immediately" and "we will hold people accountable."

Whitaker, a former U.S. attorney from Iowa who twice ran unsuccessfully for statewide office and founded a law firm with other Republican Party activists, once opined about a scenario in which Trump could fire Sessions and then appoint an acting attorney general who could stifle the funding of Mueller's probe.

In that scenario, Mueller's budget could be reduced "so low that his investigation grinds to almost a halt," Whitaker said during an interview with CNN in July 2017 before he joined the Justice Department.

In a CNN op-ed last year, Whitaker wrote, "Mueller has come up to a red line in the Russia 2016 election-meddling investigation that he is dangerously close to crossing."

Trump's relentless attacks on Sessions came even though the Alabama Republican was the first U.S. senator to endorse Trump and despite the fact his crime-fighting agenda and priorities, particularly his hawkish immigration enforcement policies, largely mirrored the president's.

He found satisfaction in being able to reverse Obama-era policies that conservatives say flouted the will of Congress, encouraging prosecutors to pursue the most serious charges they could and promoting more aggressive enforcement of federal marijuana law.

He also announced media leak crackdowns and tougher policies against opioids, and his Justice Department defended a since-abandoned administration policy that resulted in migrant parents being separated from their children at the border.

But the relationship was irreparably damaged in March 2017 when Sessions, acknowledging previously undisclosed meetings with the Russian ambassador and citing his work as a campaign aide, recused himself from the Russia investigation.

Trump repeatedly lamented that he would have never selected Sessions if he had known the attorney general would recuse himself. The recusal left the investigation in the hands of Rosenstein, who appointed Mueller two months later after Trump fired then-FBI Director James Comey.

In piercing attacks, Trump called Sessions weak and beleaguered, complained that he wasn't more aggressively pursuing allegations of corruption against Democratic rival Hillary Clinton and called it "disgraceful" that Sessions wasn't more serious in scrutinizing the origins of the Russia investigation for possible law enforcement bias - even though the attorney general did ask the Justice Department's inspector general to examine those claims.

The broadsides escalated in recent months, with Trump telling an interviewer that Sessions "never had control" of the Justice Department.

Sessions endured most of the name-calling in silence, though he did issue two public statements defending the department, including one in which he said he would serve "with integrity and honor" for as long as he was in the job.

Sessions, who likely suspected his ouster was imminent, was spotted by reporters giving some of his grandchildren a tour of the White House over the weekend. He did not respond when asked why he was there.