Whitaker’s new role includes oversight of the Russia investigation, along with the agency’s other federal investigations, including the New York prosecutors’ look into the finances of Trump and his former aides.

But Democrats and 18 state attorneys general blasted Whitaker’s appointment, pointing to his past comments in the media about the investigation. In an op-ed Whitaker wrote last year, he argued that Rosenstein should “order Mueller to limit the scope of his investigation to the four corners of the order appointing the special counsel.”

“If he doesn’t, then Mueller’s investigation will eventually start to look like a political fishing expedition,” Whitaker wrote. This would not only be out of character for a respected figure like Mueller, but also could be damaging to the President of the United States and his family—and by extension, to the country.”

Separately, in a June 2017 CNN appearance, Whitaker said that he could “see a scenario where Jeff Sessions is replaced with a recess appointment, and that attorney general doesn’t fire Bob Mueller, but he just reduces his budget to so low that his investigation grinds almost to a halt.”

Last month, though, the Justice Department Office of Legal Counsel issued an opinion supporting Whitaker’s appointment, which remains temporary.

Earlier this month, Trump appointed William Barr, who served as attorney general under the late former President George H.W. Bush, to replace Sessions. Barr is awaiting Senate confirmation. Barr has criticized Mueller as well, specifically the lack of "balance" in the team of special counsel prosecutors. The majority of investigators appointed to the special counsel's office made significant political donations to Democratic candidates and causes, with a majority also registered as Democratic voters.

But according to a Wall Street Journal report this week, Barr also sent a 20-page memo to the Justice Department calling Mueller's Russia probe "legally unsupportable" and "potentially disastrous," specifically referring any inquiries into obstruction of justice. Barr reportedly wrote that line of inquiry was "fatally misconceived."

A date for Barr's Senate confirmation hearing has yet to be set.