First, he tried blaming himself. Now, the personal lawyer for U.S. President Donald Trump is trying a new defence, one that legal experts say understates just how exposed Trump is to legal jeopardy.

It follows a potentially incriminating Trump tweet over the weekend, when the president's account made what sounded like an inadvertent confession: That the president knew his former national security adviser Michael Flynn lied to the FBI at the time he pressured FBI director James Comey to ease off investigating Flynn.

Trump's attorney John Dowd has since contended that even if the president did imply he obstructed justice, it wouldn't matter. That's because a commander-in-chief, Dowd argued, can't technically be found guilty of that crime.

Dowd, who claims to have actually written the tweet attributed to Trump, told the website Axios that the "president cannot obstruct justice because he is the chief law enforcement officer" of the United States. As such, Dowd said, Trump "has every right to express his view of any case."

Richard Painter

'None of this means the president can't weigh in on particular investigations, even though presidents rarely do that,' said Richard Painter, former ethics czar for the administration of George W. Bush. (CBC)

Certainly he does have that right, agreed former chief White House ethics lawyer Richard Painter. That's not in dispute.

"None of this means the president can't weigh in on particular investigations, even though presidents rarely do that," said Painter, who served during the George W. Bush administration.

It's also true that the president as the head of the executive branch and therefore head of the Justice Department, has the constitutional authority to decide who gets prosecuted and which staff he can terminate. There are circumstances in which the president can direct prosecutorial resources be spent elsewhere, for example.

But what amounts to obstruction occurs when a "corrupt" intent is involved, Painter said.

"If the president fires the FBI director only really because he wants to stop the investigator of his own campaign for colluding with the Russians," he said, "that is obstruction of justice."

For prosecutors, the moment came on Saturday. That's when the president's Twitter account posted this message:

It seemed to smack of an admission that Trump knew that former national security adviser Michael Flynn lied to the Federal Bureau of Investigation when the White House fired him. The day after Flynn's exit, Trump pressured Comey to ease off on the probe, according to Comey's testimony before Congress, raising new questions about Trump's motivations for firing Comey.

Susan Low Bloch, a constitutional law professor at Georgetown University who testified during the 1998 Bill Clinton impeachment proceedings, said Dowd's assessment ignores the factor that "improper motive" plays in a presidential obstruction of justice case.

Trump Special Counsel Q&A

U.S. special counsel Robert Mueller is leading an investigation into the Trump campaign's alleged ties with the Russians during the 2016 U.S. presidential election. (J. Scott Applewhite/Associated Press)

"I would say the president can unlawfully obstruct justice. The remedy if he does is to impeach and remove him with possible criminal prosecution."

That prosecution would only happen after he has left office. Another possibility is to not impeach and remove him, but to charge him when he's out of office.

The House of Representatives determines whether obstruction would fall under the umbrella of "high crimes and misdemeanours" — one of the articles of impeachment.

There is some precedent to show this would be the case. Obstruction of justice was among the three articles of impeachment filed against Richard Nixon. The former president resigned in 1974, before impeachment proceedings could begin.

Bloch said that even if Trump is impeached and removed, the vice-president could also, as was the case of Nixon's successor Gerald Ford, pardon the former president.

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Former U.S. attorney general Alberto Gonzales, who served in George W. Bush's administration, says it remains 'an unsettled question' as to whether a president is criminally liable for actions taken in office and whether the sole remedy is impeachment. (Jim Bourg/Reuters)

She said the "majority view" of scholars is that a president cannot be criminally charged while in office. There is no definitive answer, though most experts would say a president should not be indicted, tried and convicted while serving, lest it trigger practicality concerns and a constitutional crisis.

That said, "everyone agrees that if the president shot somebody, he or she would be criminally prosecutable. It's just a question of when."

The broad principle is that no person — not even the commander-in-chief — is immune to the laws of the land, said former U.S. attorney general Alberto Gonzales.

"No man should be a judge in his own case. No person is above the law, and think about this: Would the framers [of the U.S. Constitution] have intended such a scenario?"

Gonzales said it is important to remember that one of the reasons Ford gave for pardoning Nixon was his concern that Nixon would be indicted and tried for criminal acts.

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Trump jokes with Flynn as they speak at a rally on Oct. 18, 2016 in Grand Junction, Colo. (George Frey/Getty Images)

Moving toward impeachment doesn't negate the criminal process, said Jim Trusty, a former federal prosecutor who led the organized crime and gang section of the Department of Justice for six years.

"You shouldn't assume there would never be a prosecution of a sitting president still. Lack of a precedent doesn't mean lack of full-blown immunity," Trusty said. "It's clear the process with criminal activity is the impeachment process, but that doesn't by itself eliminate the possibility of a criminal case."

The argument by Trump's lawyers that he can't obstruct justice, despite the seemingly incriminating tweet, might indicate their anxiety that the president did obstruct justice.

Former federal prosecutor Jeff Cramer, managing director of the investigation firm Berkeley Research Group, suggested Dowd's claim of authorship might have been an effort to cover up a problematic admission.

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Flynn arrives for a hearing at U.S. District Court in Washington on Dec. 1. He pleaded guilty to lying to the FBI about his contacts with Russia's ambassador. (Jonathan Ernst/Reuters)

"It was a bone-headed tweet for a lawyer to send … so it's a two-pronged attack," he said. "One — he's saying the president did not just admit to a motive, I did; and two — even if he did have a motive to fire the director of the FBI, you can't prosecute the president."

The Trump lawyer's defence recalled Nixon's infamous rationale during the 1977 Frost-Nixon interviews, revealing how he perceived presidential powers.

"When the president does it, that means that it's not illegal," Nixon said.

To which Painter, the former White House ethics counsel, said Trump's team appears to be  drawing the wrong kind of inspiration.

"That's just not true, that's not the way it works," Painter said. "Nixon learned that the hard way."