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". The news comes the day after the 2018 midterm elections, which set a record for voter turnout.
Senate Minority Leader Charles Schumer says it is "paramount" that the investigation of special counsel Robert Mueller be protected by President Donald Trump's new attorney general.
Trump's relationship with Sessions frayed just weeks into his tenure, when he stepped aside from the Russian Federation investigation.
But there has also been speculation Mr Trump could try to kill or suppress the 18 month-old probe, pressing his view that Mr Mueller leads a team staffed by Democrats operating under an illegal mandate. And the department is expected at some point to receive a confidential report of Mueller's findings, though it's unclear how much will be public.
"There is some confusion among stupid people as to whether Sessions was sacked or resigned", Colbert said. The undated letter was then sent to the White House.
Could a Mueller firing be challenged on legal grounds?
In a sign of how Whitaker would handle the Mueller investigation if he does take oversight, he notably authored a piece for CNN last August, entitled, "Mueller's investigation of Trump is going to far".
That left Whitaker in charge, at least for now, though Democrats, including Rep. Nancy Pelosi and Sen.
But Prof Clark said there was no clear enforcement action, like a lawsuit, that could force Mr Whitaker to recuse himself if he declined to follow the rule.
After President Donald Trump announced the firing of Attorney General Jeff Sessions on Wednesday, to be replaced by a person who had been publicly critical of the Special Counsel Robert Mueller's investigation of ties between the Trump campaign and Russian Federation in 2016, Dean called in to CNN to give his opinion. In fact, Mr Whitaker told CNN past year that a future attorney general could limit Mr Mueller by reducing the budget "so low that his investigations grinds to nearly a halt".
The attorney general had endured more than a year of stinging and personal criticism from Trump over his recusal from the investigation into potential coordination between Russian Federation and the Trump campaign. This is the first time in U-S history a president attacks a Cabinet member as frequently and harshly in public.
He also announced media leak crackdowns and tougher policies against opioids, and his Justice Department defended a since-abandoned administration policy that resulted in migrant parents being separated from their children at the border.
Trump repeatedly lamented that he would have never selected Sessions if he had known the attorney general would recuse himself. The recusal left the investigation in the hands of Rosenstein, who appointed Mueller as special counsel two months later after Trump fired then-FBI Director James Comey.
"So I could see a scenario where Jeff Sessions is replaced with a recess appointment and that attorney general doesn't fire Bob Mueller, but he just reduces his budget to so low that his investigation grinds to nearly a halt", Whitaker said during an interview with CNN in July 2017. I believe, just today, the president indicated that he might want the Democrats in Congress to be investigated, it sounded like for criminal purposes, by the Justice Department.
Mr Whitaker is considered by some as partisan at best and a Trump loyalist at worst.