In an op-ed in August previous year he publicly urged Deputy Attorney General Rod Rosenstein - who oversees the probe - to "limit the scope of his investigation to the four corners of the order appointing him special counsel".
Susan Collins, the Republican senator from ME, 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".
U.S. Attorney General Jeff Sessions announced his resignation on Wednesday after unrelenting criticism from U.S. President Donald Trump over his recusal from an investigation into Russia's role in the 2016 presidential race.
Democrats, who won the lower house of Congress in Tuesday's midterm elections, now see Trump as close to crossing that line with the ultimate goal of covering up alleged crimes. He had frequently publicly castigated Sessions for recusing himself a year ago from the case.
"It's unfortunate that we all knew the president was going to try to get rid of Jeff Sessions", Warner, a Virginia Democrat, said.
Clovis, the 2016 Trump campaign national co-chairman, said in a May radio interview that he has been interviewed by the special counsel's team and had testified before the Mueller grand jury. Senate Minority Leader Chuck Schumer said that "protecting Mueller and his investigation is paramount".
Without Sessions' campaign or Russian Federation entanglements, there's no legal reason Whitaker couldn't immediately oversee the probe.
Matt Whitaker has taken over as acting-attorney general. Deputy Attorney General Rod Rosenstein has overseen Mueller's work.
Harry Kane: Rooney should be England captain on Wembley farewell
Former Everton and United forward Rooney scored 53 goals in 119 England appearances. "And we'll see in a few years how close I am".
Trump had enjoyed Whitaker's cable TV appearances - including one on CNN in which he suggested that the Mueller probe be starved of resources - and the two men soon struck a bond.
'Nor has a President ever before appointed an acting attorney general who is on record describing the means by which the President could curtail an ongoing investigation of the President's conduct'.
Several Republicans expressed concern about Whitaker taking over the Russian Federation probe.
Before Trump gave a news conference on Wednesday, White House Chief of Staff John Kelly called Sessions and told him that Trump wanted him out, the official said.
Here is a look at how Whitaker might stop or slow the Mueller investigation and the risks he might run in doing so.
Sessions, who was an early backer of the Trump campaign, recused himself in March 2017 from overseeing the Russian Federation investigation under a similar rule which prohibits officials from overseeing investigations into subjects with whom they have a "a personal or political relationship".
Even though Whitaker has always been skeptical of the investigation, he has never called for it to be shut down.
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".
The president can not directly fire the special counsel.