Department of Justice Chief of Staff Matt Whitaker (R) participates in a round table event with the Joint Interagency Task Force - South (JIATF-S) foreign liaison officers at the Department of Justice Kennedy building August 29, 2018 in Washington, DC.

Senate Minority Leader Chuck Schumer sent a letter Friday to President Donald Trump questioning whether the appointment of acting Attorney General Matthew Whitaker was constitutional.

Whitaker, who was then-Attorney General Jeff Sessions' chief of staff, was promoted after Sessions' ouster was revealed Wednesday. Sessions, whom Trump regularly vented his rage at in tweets and interviews, wrote in a letter to the president that he had resigned "at your request."

The announcement came less than a day after Republicans lost their majority control of the House in the midterm elections, and quickly sparked heated criticism from legal experts and Democratic lawmakers.

"Mr. Whitaker is a political appointee who is not serving in a Senate confirmed position in the Justice Department. I am not aware of any precedent for appointment of an official who has not been confirmed by the Senate to serve as Acting Attorney General," Schumer, D-N.Y.,wrote in his letter to Trump.

He also raised the possibility that Whitaker's appointment violated a clause in the U.S. Constitution requiring the president to appoint certain officials only "with the Advice and Consent of the Senate."

Some lawyers have argued that Whitaker is a "principal officer," as opposed to "inferior officers," for whom the Constitution gives the president the power to appoint without Senate confirmation.

Among those questioning the move are Neal Katyal, former acting solicitor general in the Obama administration, and George Conway, an attorney — and husband of Trump's senior advisor, Kellyanne Conway.

"A principal officer must be confirmed by the Senate. And that has a very significant consequence today," the two lawyers wrote in a New York Times op-ed Thursday. "It means that 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."

The White House did not respond to CNBC's request for comment on Conway's op-ed.

Schumer appears to adopt a similar stance in his letter to Trump. "The Attorney General, as head of the Department of Justice, is unquestionably a principal officer," Schumer said. "It is not clear to me how your appointment of an unconfirmed official as Acting Attorney General comports with this constitutional mandate – either on its face or intent."

Whitaker, 49, was indeed confirmed by the Senate — in 2004, when he became a U.S. attorney for the Southern District of Iowa. But Whitaker tendered his resignation to former President Barack Obama in 2009.

Schumer notes that Peter Keisler was appointed acting attorney general in 2007 without a separate Senate confirmation, but said that Keisler "was already serving in a Senate-confirmed position in the Department."

Whitaker was reportedly appointed under the Federal Vacancies Reform Act, which allows him to remain acting attorney general for up to 210 days. But depending on when Trump nominates a permanent replacement, Whitaker could remain in the role longer as the nomination process is carried out.

Experts have also questioned the Trump administration's use of this act, arguing that the law may not apply if Sessions was indeed fired by Trump. Even if it does, some say, the line of succession would put Deputy Attorney General Rod Rosenstein in the acting role. Rosenstein oversaw the Mueller probe after Sessions recused himself, but the DOJ said Whitaker will now oversee the investigation.

Schumer also expressed concern about Whitaker's fitness to serve in an acting capacity as the chief law enforcement official in the U.S., given his "specific expressions of bias against" special counsel Robert Mueller's Russia investigation.

Whitaker has critiqued the Mueller probe of Russian interference during the 2016 U.S. presidential election on multiple occasions. He wrote an op-ed for CNN in August 2017 arguing that if Mueller looked into the Trump family's finances "without a broadened scope in his appointment, then this would raise serious concerns that the special counsel's investigation was a mere witch hunt."

Whitaker also appeared to defend Trump campaign officials' meeting with Kremlin-connected Russians at Trump Tower in June 2016, saying: "There is no federal crime of collusion. So we're either looking at espionage charges, which seems farcical with the evidence we have now, or we're looking at campaign finance violations, but I still don't see how there's anything of value there."

In June 2017, Whitaker said on a radio show that "the truth is, there was no collusion with the Russians and the Trump campaign," Politico reported.

His remarks have led to numerous calls from Democrats and legal experts alike for Whitaker to recuse himself from the probe. But The Washington Post reported Thursday that he has no intention of recusing himself, citing people close to the new acting attorney general. Jay Sekulow, Trump's lawyer in the Mueller probe, said Thursday that Whitaker's promotion will have "no effect day-to-day" on the special counsel, Bloomberg reported.

Pelosi tweet

Schumer's letter included five questions for Trump, asking him why he chose Whitaker and whether he had discussed the Mueller probe with him, among others.

Trump appeared to respond to some of those questions in remarks to reporters Friday morning en route to Paris for events commemorating the 100th anniversary of the end of World War I.

"I don't know Matt Whitaker," the president said repeatedly. But Whitaker "is a very highly respected man. You didn't have any problems when he worked for Jeff Sessions. He's respected by law enforcement. He's a very strong law enforcement personality and person," Trump added.

Asked whether Whitaker should have been confirmed by the Senate, Trump turned the question on Mueller: "Mueller was not Senate confirmed. Why didn't they get him Senate confirmed? He should've been Senate confirmed."

Katyal and Conway argued in their op-ed, and in prior writings, that "What makes an officer a principal officer is that he or she reports only to the president. No one else in government is that person's boss. But Mr. Mueller reports to Rod Rosenstein, the deputy attorney general. So, Mr. Mueller is what is known as an inferior officer, not a principal one, and his appointment without Senate approval was valid."

Read the full text of the letter below, or download it here.