Claire Finkelstein, a law professor and director for the Center for Ethics and the Rule of Law at the University of Pennsylvania, said that even his selection to become Sessions' chief of staff — shortly after penning the CNN op-ed — should raise alarms.
"The plan clearly had been to put [Whitaker] in the Justice Department and then promote him when an opportune occasion arose," Finkelstein said.
"It's clear that he was selected because of his views on the special counsel investigation," she said. "That, again, goes to the questions of whether or not this was an orchestrated obstruction of justice campaign by Donald Trump."
The Brennan Center's Goitein noted that his "statements suggest he's opposed to a full and fair investigation by the Mueller team and has prejudged what the outcome should be, at least with respect to possible collusion between the Trump team and Russia."
Ryan Goodman, a New York University law professor and former special counsel to the Defense Department's general counsel, argued that Whitaker needed to recuse himself, citing in part Whitaker's remarks in June 2017 that "there is no criminal obstruction of justice charge to be had here" following ex-FBI Director James Comey's testimony on Capitol Hill.
Leading Democratic lawmakers, some of whom have already called on Congress to pass legislation that would protect the special counsel, agreed that Whitaker should not oversee the investigation.
Democratic Reps. Jerrold Nadler and Adam Schiff, who are considered likely to become chairmen of House committees next year, expressed concerns about Sessions' resignation and Whitaker's appointment.
The timing of Whitaker's appointment — less than a day after Democrats won back majority control of the House — was also noteworthy to some legal experts.
Schiff, who could lead the House Intelligence Committee and revive its own probe of Russian election meddling, said "Whitaker and any nominee must commit" to protecting Mueller and the independence of the Justice Department.
"It is impossible to read Attorney General Sessions' firing as anything other than another blatant attempt by @realDonaldTrump to undermine & end Special Counsel Mueller's investigation," House Minority Leader Nancy Pelosi said before calling for Whitaker to recuse himself.
Nadler, who is expected to lead the House Judiciary Committee, raised the possibility that Trump was firing Sessions and hiring Whitaker "for the purposes of subverting the rule of law and obstructing justice."
The new Congress, which will see Democrats controlling the House committees and possibly exerting oversight powers on the Trump administration, will not take effect until Jan. 3, 2019.
The Republicans who still hold the House majority until that date are "not likely to exercise" congressional oversight in the interim, said William Heffernan, law professor at John Jay College of Criminal Justice.
Currently, Whitaker is "in a position to seriously undermine the Russia investigation," argued Goodman and Harvard law professor Alex Whiting.
"His repeated expression of hostility to the Mueller investigation makes it impossible for the public to have confidence in his ability to exercise the necessary prosecutorial judgment," Goodman wrote in a separate piece calling for Whitaker's recusal.