President Trump's decision to replace Attorney General Jeff Sessions with Matthew Whitaker is facing new questions, with two prominent attorneys — including the husband of White House counselor Kellyanne Conway — suggesting that it may be illegal.

Why it matters: Axios spoke to multiple legal experts and former Justice Department officials who say they can't remember a similar case where someone not confirmed by the Senate has been named as acting attorney general. They have different interpretations of the laws, but they agree that the naming of Whitaker is in uncharted legal territory and leaves room for challenges to the legality — and constitutionality — of Trump's actions.

What they're saying: Attorneys Neal Katyal and George Conway called Trump's action illegal in an op-ed published in the New York Times Thursday afternoon. They say the Constitution requires that anyone serving as attorney general be confirmed by the Senate.

The details: While the Vacancies Reform Act gives the president the power to choose a temporary replacement for attorney general (as long as the attorney general was not fired), the Constitution dictates that anyone serving in a "principal role" must be confirmed by the Senate.

Here are the legal and constitutional issues raised by the Whitaker appointment:

1. The Attorney General Succession Act, Section 508, stipulates that when there is a vacancy in the office of the attorney general, the deputy attorney general — currently Rod Rosenstein — can serve as the attorney general. Next in line would be the associate attorney general.

2. The Federal Vacancies Reform Act of 1998 allows the president to choose any senior DOJ official to serve as an acting attorney general as long as that person has already served in a high-level position for 90 days. Whitaker qualifies for the temporary position under this Act.

There have been arguments on both sides over whether the act supplants the AG succession law, John Bies, former deputy assistant attorney general in the office of legal counsel, told Axios.

"I think it’s pretty clear under the terms of the statute itself that [Trump] has authority ... to appoint someone who is a senior DOJ official," he said.

"The Vacancy Act has precedence [over the Succession Act] when the president picks someone under that procedure," another DOJ lawyer texted us.

3. The Constitution, Article II, Section 2, Clause 2 is the big caveat that Conway and Katyal argue leaves zero wiggle room for Whitaker. The Constitution specifies that "principal officers" must be appointed with the advice and consent of the Senate.

"We cannot tolerate such an evasion of the Constitution’s very explicit, textually precise design. Senate confirmation exists for a simple, and good, reason. Constitutionally, Matthew Whitaker is a nobody. His job as Mr. Sessions’s chief of staff did not require Senate confirmation. ... For the president to install Mr. Whitaker as our chief law enforcement officer is to betray the entire structure of our charter document."

— Conway and Katyal in their NYT op-ed

The other side: Justice Department spokeswoman Sarah Isgur Flores emailed, "The VRA was passed in 1998 and Acting Attorney General Whitaker's appointment was made pursuant to the procedures approved by Congress."

One big question: Whether a temporary position is considered a principal role. In October 2016, Justice Clarence Thomas argued that even a temporary appointment of someone who was not confirmed by the Senate would violate the clause in the Constitution.

Yes, but David Rivkin, a constitutional lawyer who has served in previous administrations, told us: "When you put someone in an acting capacity, that person doesn’t occupy an official office, so the appointments clause doesn’t apply in this case at all. It’s temporary."

The bottom line: Even John Yoo, a law professor at the University of California-Berkeley who helped the George W. Bush administration draft its expansive claims to executive power, says the Whitaker appointment may be out of line.

"The Constitution says that principal officers must go through appointment with the advice and consent of the Senate. In Morrison v. Olson, the Supreme Court made clear that the Attorney General is a principal officer. Therefore, Whittaker cannot serve as acting Attorney General despite the Vacancies Act (which does provide for him to be acting AG) — the statute is unconstitutional when applied in this way."

— John Yoo in an email

Editor's note: This story has been updated to reflect a new quote from DOJ spokeswoman Sarah Isgur Flores.