The appointment generated immediate controversy given Mr Whitaker's previous public comments criticising the probe. Democrats worry that firing Sessions is a path to removing special counsel Robert Mueller and trying to end the probe.
"It does not take a lawyer or even a former federal prosecutor like myself to conclude that investigating Donald Trump's finances or his family's finances falls completely outside of the realm of his 2016 campaign and allegations that the campaign coordinated with the Russian government or anyone else", he wrote. He was a vocal critic of Mr Mueller, arguing in an op-ed for CNN previous year that the probe should not go into the finances of Mr Trump and his family members.
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.
Her words were echoed by Democratic party Senate leader Chuck Schumer, who added: "Clearly, the president has something to hide."
For more than a year, Trump has repeatedly attacked Sessions over his recusal from the Russian Federation investigation, saying he wouldn't have installed him in the post had he known he would recuse himself from the probe.
Mr Trump has at various times belittled Mr Sessions as "beleaguered", "VERY weak", and "DISGRACEFUL".
"He took the job and then he said, "I'm going to recuse myself.' I said, 'What kind of a man is this?'" Trump said this year in a Fox News interview".
A handful of Republicans joined in the chorus, including Utah senator and former presidential candidate Mitt Romney who wrote on Twitter it was "imperative. that the Mueller investigation proceeds to its conclusion unimpeded".
"Sessions noted in his resignation letter to the president that he was stepping down 'at your request'".
A personal connection to a Mueller witness is unlikely to sway Whitaker or the White House when it comes to recusal, as Deputy Attorney General Rod Rosenstein supervised the probe despite being a witness himself.
"Interference with the special counsel's investigation would cause a constitutional crisis and undermine the rule of law".
Trump has said for months that he wished to replace Sessions, but lawmakers and administration officials believed that firing the attorney-general before the midterm elections would have had negative consequences for Republicans in tight races.
"He's political to his core", a friend said of Whitaker.
Would he would fire special counsel Robert Mueller?
Trump's running commentary criticizing Sessions and the Justice Department injected more turmoil into an agency that was still reeling from the president's firing of FBI Director James Comey in May 2017.