Matt Whitaker, Mr Sessions’s former chief of staff, will now oversee Robert Mueller’s probe into 2016 Russian election meddling despite repeatedly criticising the scope of the investigation.
Mr Whitaker was a legal commentator before joining the Justice Department last year, and he echoed the US president by warning the probe could become a “witch hunt” and that Mr Trump’s finances should not be investigated, calling that a “red line”.
His new role as acting attorney general gives him the power to reject requests by Mr Mueller, the special counsel, as well as end parts of his investigation, fire him and even keep secret his final report.
The personnel changes, which saw Mr Trump ask Mr Sessions to resign as attorney general just hours after the midterm elections, have triggered fears that the president is moving to restrict the Russia probe.
All 17 Democrats who sit on the House Judiciary Committee penned a letter to the Republican chairman warning the country was “in the throes of a constitutional crisis”.
They called for “emergency hearings” and that “all relevant materials” to the firing be preserved.
A protest in New York City drew several hundred people calling for the protection of Mr Mueller’s investigation. Similar rallies were being held across the country. Organisers say the appointment of Mr Whitaker is a “deliberate attempt to obstruct the special counsel’s investigation”.
Democrats will take control of the committee in January after winning back the House of Representatives in the midterms but for now are reliant on the Republicans to approve hearings, which seems unlikely.
The Russia probe, which is looking into whether Trump campaign figures conspired with the Kremlin in the 2016 election and if the president obstructed justice, has dogged Mr Trump’s first two years in office.
Mr Mueller, appointed to lead the investigation early last year, is still hoping to interview the president – something the White House has been resisting for months.
Mr Trump was infuriated by Mr Sessions’s decision to recuse himself from overseeing the probe in March 2017, often complaining that he needed an attorney general who would protect him.
In Mr Whitaker, Mr Trump has found a man who echoes the president’s attacks on Mr Mueller.
The 49-year-old former college American football star wrote that Mr Mueller had come “dangerously close to crossing” a “red line” by looking into the finances of Mr Trump in an opinion piece for CNN last year.
He also shared an article with the headline ‘Note to Trump’s lawyer: Do not co-operate with Mueller lynch mob’ on Twitter, adding himself that it was “worth a read”.
And on funding for the Russia probe, he said on CNN in July 2017: “I could see a scenario where Jeff Sessions is replaced, it would [be a] recess appointment and that attorney general doesn’t fire Bob Mueller but he just reduces his budget to so low that his investigations grinds to almost a halt.”
The comments have raised suspicions that Mr Whitaker could try to limit Mr Mueller’s investigation.
The next few weeks are expected to be critical for the Mueller investigation.
CNN reported that Mr Mueller’s team has begun writing its final report and there are signs the special counsel’s team are looking into Roger Stone, a political consultant and ally of Mr Trump.