As the special counsel investigation inches deeper into the White House, President 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 advisor, Michael Flynn, from an FBI investigation and then fired FBI Director James B. Comey to block the inquiry. 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 Justice Department.
Trump's potential legal jeopardy is separate from any political liability: obstruction-of-justice charges were among the articles of impeachment filed against both President Nixon, who resigned in disgrace, and President 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."
Turley 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 S. Mueller III, who is investigating 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, a retired three-star Army general who was one of Trump's closest advisors 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 advisor 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 said he had written it, not the president. Either way, it cast a new spotlight on Senate testimony from Comey, the former FBI director who 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 advisor.
"I hope you can see your way clear to letting this go," Comey recalled the president saying.
Trump denied on Sunday telling Comey to stop investigating Flynn. Raj Shah, a White House spokesman, referred questions to Dowd, who declined to comment. Trump has previously said that he's not personally under investigation.
Mueller has not said one way or another, but all indications are his investigation is far from over. So far he has charged four people and two of them — Flynn and another campaign aide, George Papadopoulos — have pleaded guilty and agreed to assist the FBI.
Jed Shugerman, a law professor at Fordham University, said Dowd's comment showed he was 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," he said.
Sen. Dianne Feinstein, a California Democrat and ranking member on the Senate Judiciary Committee, told NBC's "Meet the Press" on Sunday that she sees "a case of obstruction of justice" in Trump's decision to fire Comey.
"It is my belief that that is directly because [Comey] did not agree to 'lift the cloud' of the Russia investigation," she said. "That's obstruction of justice."
Harvard Law School professor emeritus Alan Dershowitz told Fox News on Monday that Feinstein "simply doesn't know what she's talking about" because Trump was within his rights to fire Comey.
"There's never been a case in history where a president has been charged with obstruction of justice for merely exercising his constitutional authority," Dershowitz said. "That would cause a constitutional crisis in the United States."
Solomon Wisenberg, a lawyer who worked with the Office of Independent Counsel that investigated President Clinton, said obstruction cases generally required a suspect to have been engaged in a pattern of activity intended to impede an investigation or have been involved in the original criminal activity that's under scrutiny.
Nixon's downfall was accelerated by a White House recording that revealed he was involved in a plan to thwart an FBI investigation into the illegal bugging of Democratic campaign offices at the Watergate. Obstruction was the first of three articles of impeachment approved by the House Judiciary Committee. He resigned in 1974 before the impeachment process advanced to the full House.
Clinton was impeached by a Republican-controlled House in 1998 after his affair with Monica Lewinsky, a White House intern, was revealed. One of the allegations was obstruction of justice. The effort to remove Clinton from office stalled in the Democratic-controlled Senate.
Richard J. Davis, a lawyer who served on the Watergate prosecution task force, said both Democrats and Republicans have agreed that obstruction of justice is an impeachable offense.
"I remember a lot of debate about whether a sitting president could or should be indicted, but none of it was premised on the notion of whether the president could commit the crime of obstruction of justice," he said. "He clearly can."