Before Whitaker joined to the Justice Department, he wrote an opinion piece for CNN in August 2017 criticizing the investigation by special counsel Robert Mueller.
The president also warned the Democrats against investigating him, after taking over the House in the midterm elections.
If Mr Whitaker chose to fire Mr Mueller, he would need to inform the special counsel in writing of the specific reason for his termination.
According to Dean, Trump won't necessarily fire special counsel Robert Mueller, who is investigating possible collaboration between the Trump campaign and Russian Federation during the 2016 election - but may be aiming "to undercut [Mueller] by the people around him".
Democrats have slammed Whitaker's appointment to replace Sessions, arguing that Trump is trying to undermine Mueller and demanding that Whitaker recuse himself from supervising the Mueller investigation due to his writings and public comments attacking the investigation. Sessions also said in a letter to Trump he resigned at the president's request.
"I am not concerned about anything with the Russian investigation, because it is a hoax", he said.
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Mitt Romney, who won the Utah Senate race on Tuesday echoed Ms Collins' words and wrote in a tweet: "It is imperative that the important work of the Justice Department continues, and that the Mueller investigation proceeds to its conclusion unimpeded".
The announcement set off a frenzy inside the White House, with Trump directing his White House counsel to call Sessions beforehand and urge him not to step aside.
Trump and Sessions had a falling out after the attorney general recused himself from Mueller's investigation.
But his departure was anticipated since early this year, after he endured withering and repeated criticism from the president over the legally troubled ban on Muslim travellers Trump sought when he came into office, and over the Mueller probe. Democrats had promised to reopen the House Intelligence Committee's investigation into Trump, to investigate Trump's finances, and to obtain his tax returns. In that scenario, Mueller's budget could be reduced "so low that his investigation grinds to nearly a halt", Mr. Whitaker said during a July 2017 interview with CNN.
He found satisfaction in being able to reverse Obama-era policies that conservatives say flouted the will of Congress, encouraging prosecutors to pursue the most serious charges they could and promoting more aggressive enforcement of federal marijuana law. Trump has long made it clear he did not agree with Sessions' decision to recuse himself from the investigation, berating him in an at times brutal public fashion. He also announced media leak crackdowns, tougher policies against opioids and his Justice Department defended a since-abandoned administration policy that resulted in parents being separated from their children at the border.
Mr Whitaker is considered by some as partisan at best and a Trump loyalist at worst.