"Whitaker was Sessions' chief of staff and had been considered for a variety of jobs in the Trump administration, including the No. 2 post at Justice or as White House counsel".
Those concerns are now focused on the future of the Mueller probe, which began as a look into alleged links with Russians seeking to disrupt the election and expanded into an investigation of billionaire Trump's murky finances, including his business ties to Russia.
He served as the U.S. attorney in the state's southern district, was elected state attorney general in 1994 and then to the Senate in 1996. Previously he was a conservative legal commentator who was critical of the scope of Mueller's probe.
Democratic Representative Jerrold Nadler, who is in line to become chairman of the House Judiciary Committee in January, said it is "wholly inappropriate" for Whitaker to take over the Mueller probe and that Rosenstein should continue overseeing it.
"For me, I saw the impact of marijuana on our border", he said, presumably referring to his time as a United States attorney.
The Democrats, who won the House in the mid-terms, have vowed to protect it. He tweeted that "we will be holding people accountable".
The switch, announced on Trump's Twitter feed, provoked instant consternation across Washington, where politicians from both sides of the aisle have long warned that political interference in Mueller's work can not be tolerated. "Special Counsel Mueller must be allowed to complete his work without interference-regardless of who is AG".
Separately, Justice Department prosecutors in NY 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. Author of "Kompromat: How Russia Undermined American Democracy".
Sessions took the lead in imposing Trump's travel ban targeting a number of mostly Muslim nations, canceling the so-called Dreamer program that let the children of undocumented immigrants remain in the US and forcing the separation of migrant families as part of a border crossing crackdown. The undated letter was then sent to the White House.
"Most importantly as my time as attorney general, we have restored and upheld the rule of law", he wrote, while also thanking the Republican president.More news: Demi Lovato's First Instagram After Rehab Is a Voting Photo
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Trump announced the resignation and Whitaker's assignment on Twitter.
"I could see a scenario where Jeff Sessions is replaced with a recess appointment". But Mr Sessions's replacement will have the power to do so, or end the inquiry. So it came as little surprise when Sessions' resigned the day after the midterms were over.
Sessions had recused himself from the investigation, because of his own contacts with Russians during the 2016 campaign, instead handing responsibility for Mueller to his deputy Rod Rosenstein. "I wanted to stay uninvolved".
Any action by Mr Whitaker could potentially be challenged under the Vacancies Reform Act Trump used to appoint his acting attorney general.
Can Whitaker be forced to recuse himself? Trump has already fired James Comey, the FBI director originally overseeing the investigation.
Whitaker's ascendance to the top of the Justice Department shows how much loyalty means to Trump. The president has long regarded Whitaker as his eyes and ears inside a department that he considers an enemy institution.
A former college football player and U.S. attorney, Whitaker has been a frequent White House visitor and served as what one White House aide called a "balm" on the relationship between the President and the Justice Department.
Marc Lotter, former spokesman for Vice President Pence, former special assistant to President Trump.
In pushing out his attorney-general, the President cast aside one of his earliest and strongest supporters.
Sessions - who bonded with Trump over their populist views on trade and immigration - became the first sitting senator to endorse Trump in February 2016 when he announced his support of the NY businessman's then-underdog campaign.
A political analyst in the state predicted Sessions would run for the seat.
In September, Trump said, "I don't have an attorney general". His recusal was one of his first public acts as attorney-general.