Mr Sessions, a former senator representing Alabama, was among the early loyalists of Mr Trump before the relationship was soured shortly after he took office as Attorney General in 2017.
That's because Sessions had recused himself, calling it inappropriate that he lead an inquiry into Trump's 2016 presidential campaign of which he was a part.
Mr Whitaker has publicly criticised Mr Mueller's conduct several times since the inquiry began in May previous year and taken the side of Mr Trump, who has dismissed it...
But it's not typical for chiefs of staff to replace the head of an agency when that head resigns, as former CIA staffer and NBC contributor Ned Price pointed out.
If Sessions' departure was an "opening move" by Trump to meddle in Mueller's investigation, Hoyer said in a statement, "the president must be held accountable". "That's why Cabinet Secretaries have Deputies".
Justice Department rules on special counsels set boundaries on how Mueller could be removed.
In July 2017, he said during an interview on CNN that he could envision a scenario under which a temporary attorney general doesn't fire Mueller but rather "just reduces his budget to so low that his investigations grinds to nearly a halt". Sessions reportedly resigned at the request of the president.
Mitt Romney, the Republican Party's 2012 presidential nominee who won the Utah race on Tuesday to become a USA senator, also said Mueller's probe should not be affected by Sessions' dismissal.
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She even threw in a #friendshipneverends for good measure. "Nothing has changed, I've always been Granny Spice with biscuits in my handbags", she said.
Gonzales, who was the attorney general in George W. Bush's administration from 2005 to 2007, said he doesn't know Whitaker, his qualifications or motivations, but there's a lot that calls into question his appointment.
"Attorney General Jefferson Sessions was a national disgrace, NORML hopes he finds the time during his retirement to seek treatment for his affliction of 1950's reefer madness", Erik Altieri, executive director of the National Organization for the Reform of Marijuana Laws, said in a press release.
The relentless attacks on Sessions came even though the Alabama Republican was the first USA senator to endorse Trump and despite the fact that his crime-fighting agenda and priorities - particularly his hawkish immigration enforcement policies - largely mirrored the president's.
Trump said he will nominate a permanent successor for Sessions at a later date. "We're mobilizing immediately to demand accountability, because Trump is not above the law".
The president hasn't been charged with any wrongdoing, but federal prosecutors have said that a grand jury investigation is ongoing and it targets unspecified "others".
US Representative Earl Blumenauer, of OR, called the move a "major step forward for marijuana reform", also noting that US Representative Pete Sessions, of Texas, who has obstructed votes on marijuana-related legislation as chair of the House Rules Committee, was defeated in Tuesday's midterm elections.
Whitaker is expected to assume oversight of Mueller's Russian Federation investigation.
But Senate Judiciary Chairman Chuck Grassley praised Whitaker.
A look at the responsibilities of the Justice Department's top two leaders and the special counsel running the department's most consequential and politically sensitive investigation.
An ever-more exasperated Trump ordered him to give up the mic and Acosta refused, with Trump branding him an "enemy of the people" and a "rude, awful person".