Washington(CNN) The Justice Department is expected to issue an opinion from the Office of Legal Counsel on Tuesday that will defend Matthew Whitaker's appointment as acting attorney general under federal law, according to a source familiar with the matter.

Almost immediately after President Donald Trump appointed Whitaker last week, various high-profile lawyers, professors and former Justice officials have weighed in on the legality of the move and more formal legal challenges are widely expected soon.

Some legal scholars -- as well as George Conway, the husband of Trump counselor Kellyanne Conway -- have said they thought Whitaker's appointment is unconstitutional because he is not a Senate-confirmed official. Yet others have argued that the appointment is valid because he's only serving on a temporary basis under the Federal Vacancies Act.

OLC serves as the chief interpreter of federal law for the administration, but its conclusions are not binding on federal courts, and Tuesday's opinion is unlikely to abate the brewing controversy over Whitaker's appointment.

Top Democrats on Capitol Hill have slammed Trump's decision to appoint Whitaker as acting attorney general, skipping over Deputy Attorney General Rod Rosenstein -- the next Justice official in the succession line who is Senate-confirmed and was previously overseeing special counsel Robert Mueller's investigation into Russian interference in the 2016 election. While Rosenstein is still managing the probe on a day-to-day basis, Whitaker's critics have worried that he could take steps to curtail Mueller's work given his past fierce criticism of the probe.

Others have demanded that Whitaker recuse himself from oversight of the investigation.

Justice Department spokeswoman Kerri Kupec told CNN Monday that Whitaker will follow all ethics rules.

"Acting AG Whitaker is fully committed to following all appropriate processes and procedures at the Department of Justice, including consulting with senior ethics officials on his oversight responsibilities and matters that may warrant recusal," Kupec told CNN.

The department declined to get into specifics on whether he has initiated that review. As of last week, CNN reported that had not yet happened, and Whitaker has given no indication he believes he needs to step aside.