In the aftermath of the midterm elections, in which Democrats took control of the House of Representatives, Mr Trump ousted Mr Sessions and replaced him with Matthew Whitaker, a Trump loyalist who has previously been critical of the special counsel investigation into whether the Trump campaign colluded with Russia.
Democrats said they feared the decision to appoint Mr Whitaker was the prelude to a decision by Mr Trump to terminate the investigation by Mr Mueller, which the president has constantly described as a “witch hunt”.
While most Republicans remained silent about the firing of Mr Sessions – among them South Carolina senator Lindsey Graham, who said last year there would be “hell to pay” if the attorney general was got rid of – three senior members of the party urged Mr Trump to allow Mr Mueller complete his probe.
Mitt Romney, who challenged Barack Obama for the White House in 2012, and who this week was elected to the senate from Utah, wrote on Twitter: “I want to thank Jeff Sessions for his service to our country as attorney general."
He added: “Under acting attorney general Matthew Whitaker, it is imperative that the important work of the Justice Department continues, and that the Mueller investigation proceeds to its conclusion unimpeded.”
Susan Collins, the Republican senator from Maine, and one of the party’s more moderate elected politicians, also praised Mr Sessions, saying he was “a leader of integrity who served our country well”.
“It is imperative that the administration not impede the Mueller investigation,” she said. “I’m concerned Rod Rosenstein will no longer be overseeing the probe. Special Counsel Mueller must be allowed to complete his work without interference – regardless of who is AG.”
Lamar Alexander, Republican senator from Tennessee, issued perhaps the stormiest warning, saying the Senate would not confirm an attorney general who sought to stop the Russia probe.
But those three were the only senior Republicans to speak out. Senator Graham, who offered fierce defence of both Mr Sessions, and Mr Mueller’s probe just 12 months ago, was criticised for merely saying: “I look forward to working with President Trump to find a confirmable, worthy successor so that we can start a new chapter at the Department of Justice and deal with both the opportunities and challenges our nation faces.”
Newly emboldened Democrats have said any attempt to terminate Mr Mueller’s investigation would amount to a constitutional crisis.
Nancy Pelosi, who leads Democrats in the House of Representatives, called the decision a “blatant attempt” to end or impede the investigation.
“It is impossible to read Attorney General Sessions’ firing as anything other than another blatant attempt by @realDonaldTrump to undermine & end Special Counsel Mueller’s investigation,” tweeted Ms Pelosi, who is said to be speaker of the House, after the Democrats victory this week.
The party’s leader in the Senate, Chuck Schumer, said: “Our paramount view is that any attorney general, whether this one or another one, should not be able to interfere with the Mueller investigation in any way.”
While Mr Trump assigned oversight of the probe to Mr Whitaker, who last year wrote an opinion piece for CNN calling on Mr Mueller limit the scope of his probe, it is not clear Mr Trump would at this point benefit from shutting down the probe.
Having had 18 months to investigate matter, and having brought indictments against or secure guilty pleas from 32 people and three companies – among them four former advisors to Mr Trump – many in Washington believe Mr Mueller must be close to completing his investigation anyway. Mr Trump has insisted he believes it will clear him on collusion.
Asked at a chaotic White House press conference on Wednesday whether he intended to fire Mr Mueller, he said he could have done long ago if he had intended to.
“I could fire everybody right now, but I don’t want to stop it because politically I don’t like stopping it,” Mr Trump said. “It’s a disgrace. It should have never been started because there was no crime.”