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(CNN) —  

There is a growing sense of concern inside the White House over the negative reaction to Matthew Whitaker being tapped as acting attorney general after Jeff Sessions’ abrupt firing.

Whitaker, who was Sessions’ chief of staff, has faced criticism since Wednesday afternoon’s announcement for his previous comments on special counsel Robert Mueller’s investigation.

Several senior officials told CNN they were surprised by the criticism, and believe it could potentially jeopardize Whitaker’s chances of remaining in the post if it continues to dominate headlines.

Whitaker is expected to take over oversight of Mueller’s investigation into Russian interference in the 2016 election and whether Trump campaign associates colluded with Russia. He has given no indication he believes he needs to step aside from overseeing the probe, according to one person familiar with his thinking, a belief echoed by White House officials. And a source close to the President told CNN that the idea of Whitaker ending or suppressing the Russia probe is not an option as of now.

But Whitaker has previously expressed deep skepticism about the probe, including arguing in a 2017 CNN op-ed that Mueller was “dangerously close to crossing” a red line following reports that the special counsel was looking into Trump’s finances and calling Mueller’s appointment “ridiculous” and “a little fishy” in a 2017 appearance on the “Rose Unplugged” radio program.

Whitaker also spoke about the investigation in numerous other radio and television appearances, including CNN, where he was a legal commentator.

It was not widely known among White House staff that he’d commented repeatedly on the special counsel’s investigation in interviews and on television – which is ironic given that this is what drew President Donald Trump to him and raises continued questions over the depth of the administration’s vetting process.

Sam Clovis, a 2016 Trump campaign national chairman who has close ties to Whitaker, encouraged him to get a regular commentary gig on cable television to get Trump’s attention, according to friends Whitaker told at the time. Whitaker was hired as a CNN legal commentator last year for several months before leaving the role in September 2017 to head to the Justice Department.

Along with the breadth of his previous comments on the investigation, there have been questions about the legality of Whitaker’s appointment.

George Conway, the husband of White House counselor Kellyanne Conway, co-authored a New York Times op-ed published Thursday that called the appointment “unconstitutional.”

The Appointments Clause of the Constitution, Article II, Section 2, Clause 2, Conway wrote, “means Mr. Trump’s installation of Matthew Whitaker as acting attorney general of the United States after forcing the resignation of Jeff Sessions is unconstitutional. It’s illegal. And it means that anything Mr. Whitaker does, or tries to do, in that position is invalid.”

Whitaker’s standing ultimately depends on the President. But continued negative coverage will get Trump’s attention.

CNN’s Veronica Stracqualursi, Paul LeBlanc, Andrew Kaczynski, Jeremy Herb, Jeff Zeleny, Pamela Brown, Evan Perez and Laura Jarrett contributed to this report.