Whitaker was hired by Sessions to be his chief of staff in September 2017.
Whitaker was Sessions' Chief of Staff and, before that, an attorney with conservative watchdog group Foundation for Accountability and Civic Trust (FACT).
Whitaker is expected to take charge of the the Russian Federation investigation and special counsel Robert Mueller from Deputy Attorney General Rod Rosenstein.
In a long-expected but still shocking move, President Trump fired Attorney General Jeff Sessions just hours after the 2018 midterm elections. Trump tweeted. "A permanent replacement will be nominated at a later date". Sessions' exit also opens up the question of who will permanently fill the position.
The president's frustrations with his attorney general have at times publicly boiled into what appeared to be anger over many issues related to the Justice Department's Russian Federation election meddling probe.
It appeared to be part of a summer 2017 campaign by Whitaker - then reportedly on the shortlist to replace Don McGahn as White House counsel - to portray himself as a hardliner on the Mueller probe.
He is reported to have frequented the Oval Office in the last few years and have an "easy chemistry" with Trump, serving as a "balm" in the tenuous relationship between Trump and Sessions that has finally come to a close.
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Mr. Sessions had recused himself from that matter, causing much of the break with Mr. Trump.
"The Senate must demand that any nominee for attorney general must commit to not interfere in the special counsel investigation, and continue to have Robert Mueller operate under the special counsel regulation", Romero's statement said.
In his resignation letter, Sessions wrote: "Dear Mr President". But Sessions also thought that by staying in the job, he had protected the integrity of the investigation, the person said. He will serve our Country well ...
Whitaker added in the piece that "it is time for [Rod] Rosenstein...to order Mueller to limit the scope of his investigation". It is not immediately clear if Whitaker will immediately take control of the investigation.
Mr Rosenstein in turn appointed Mr Mueller, a former Director of the Federal Bureau of Investigation.