Senate Intelligence Committee Vice Chair Mark Warner said Thursday on "Your World" that with President Trump's appointment of former Iowa federal prosecutor Matthew Whitaker as acting attorney general, the probability of the Mueller probe being tampered with will increase.
Congressional Democrats, concerned about protecting Mueller, called on Whitaker to recuse himself from overseeing the investigation in its final but potentially explosive stages.
House Democratic leader Nancy Pelosi called it a "blatant attempt" to undermine the Russian Federation probe, while Senate Democratic leader Chuck Schumer tweeted that "clearly, the president has something to hide".
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".
She said the investigation into alleged Russian Federation collusion has been winding down for weeks prior to Sessions stepping down.
US Attorney General Jeff Sessions has endured more than a year of criticism from Donald Trump over the Mueller inquiry.
"We will not allow Trump to get away with obstructing this criminal investigation, and we will not allow Congress to abdicate its responsibility under the Constitution".
Newly emboldened Democrats have said any attempt to terminate Mr Mueller's investigation would amount to a constitutional crisis. And the department is expected to receive a confidential report of Mueller's findings, though it's unclear how much will be public. The former attorney general submitted a letter of resignation at the president's "request".
Protesters call for protection of Russian Federation probe after Sessions resigns
He could also refuse to sign off on a subpoena of Trump if the president refuses to voluntarily speak to investigators. Attorney General Elliot Richardson and his deputy William Ruckelshaus resigned rather than comply with Nixon's order.
Rosenstein remains at the department and could still be involved in oversight.
"Americans must have answers immediately as to the reasoning behind" Trump's move to remove Sessions, he said.
The department's No. 2, Deputy Attorney General Rod Rosenstein, took over and appointed Mueller after Trump fired then-FBI Director James Comey in May 2017.
That left Whitaker in charge, at least for now, though Democrats, including Rep. Nancy Pelosi and Sen. The moment the Democrats clinched the required 23 seats to retake the lower chamber; they were readying the various probes into this presidency-and the left wing harassment of this administration is about to begin.
In an interview with CNN in 2017, he argued that the inquiry could be kneecapped if the attorney general "just reduces his budget so low that the investigation grinds nearly to a halt".
The announcement, which came in a tweet, comes after months of speculation that Sessions could be on his way out after recusing himself from the investigation into Russian interference in the 2016 presidential elections. After Pawlenty's bid fizzled, he served in the 2012 and 2016 presidential campaigns of Rick Perry, the former Texas governor who is now energy secretary.
He had also pressed to reauthorize a controversial secret domestic spying program over the Justice Department's protests.