WASHINGTON — As the special counsel investigation inches deeper into the White House, President Donald Trump has repeatedly insisted that he has done nothing wrong, denying any collusion with Russia and decrying the allegations as a “hoax.”

On Monday, one of his lawyers offered a backup argument as Trump faces new questions about whether he improperly tried to shield his former national security adviser, Michael Flynn, from an FBI investigation and then fired FBI Director James Comey to block the probe. Interfering with the case could be construed as obstructing justice, a potential federal crime.

It’s impossible, the lawyer said, for the president to obstruct justice under the law. The reason? He’s too powerful.

Trump “cannot obstruct justice because he is the chief law enforcement officer under (Article II of the Constitution) and has every right to express his view of any case,” John Dowd, a lawyer for the president, told Axios, a news website. Article II details the president’s authority over the executive branch, which includes the Department of Justice.

Trump’s potential legal jeopardy is separate from any political liability: Obstruction of justice charges were among the articles of impeachment filed against President Richard Nixon, who resigned in disgrace, and President Bill Clinton, who served out his term.

But Dowd’s claim sparked a sharp debate among legal scholars and others as to how far a president can stretch his constitutional powers without breaking the law and the extent of a president’s legal immunity in office. While the Constitution lays out conditions under which a president can be impeached and removed by Congress, it does not say whether the president can be prosecuted. Neither does federal law. Courts have never ruled on the issue.

“It’s an unanswered question,” said Susan Low Bloch, a Georgetown University law professor who doubts a sitting president can be indicted on criminal charges. “We’ve never done it.”

Jonathan Turley, a George Washington University law professor, said the question “has been a parlor game for constitutional scholars for decades.”

“When we hang out we talk about bizarre scenarios, and this is one of the favorites,” he said.

Turley said he believes a president could be indicted, noting that federal judges aren’t immune from criminal charges. “Why assume a limitation with regards to presidents when it clearly doesn’t exist for other impeachable officials?” he asked.

Dowd’s comment came after special counsel Robert Mueller, who is probing whether anyone in Trump’s orbit helped Russian operatives interfere with last year’s presidential campaign, rocked the White House by making a plea deal with Flynn, who was one of Trump’s closest advisers during the campaign.

On Friday, Flynn pleaded guilty to lying to the FBI in January about his contacts with Sergey Kislyak, then the Russian ambassador, during the presidential transition. On Saturday, Trump tweeted that he had fired Flynn as national security adviser in February in part because he knew Flynn had lied to federal agents.

Trump’s claim for the first time that he knew Flynn may have committed a felony was so extraordinary that Dowd later claimed he had written the tweet. Either way, it cast a new spotlight on Senate testimony from Comey, whom Trump fired in May.

The day after Flynn was forced out of the White House, Comey later testified, Trump asked him to go easy on his former national security adviser.

Trump denied Sunday telling Comey to stop investigating Flynn.

All indications are that Mueller’s investigation is far from over.

Jed Shugerman, a law professor at Fordham University, said Dowd’s comment shows he is concerned about Trump’s legal vulnerability.

“Dowd and others have to argue that a president can’t obstruct justice because the facts are increasingly stacked against them,” Shugerman said.

Harvard Law School professor emeritus Alan Dershowitz told Fox News on Monday that “there’s never been a case in history where a president has been charged with obstruction of justice for merely exercising his constitutional authority. That would cause a constitutional crisis in the United States.”

chris.megerian@latimes.com