It was a darker-than-usual threat by U.S. President Donald Trump, one that has alarmed First Amendment advocates and prompted a senator to wonder whether Trump has breached his oath of office.

In words that intensified his anti-media scorn, Trump this week took aim once again at the "fake news," declaring in tweets that some networks should have their licences "challenged" or even "revoked."

Network news has become so partisan, distorted and fake that licenses must be challenged and, if appropriate, revoked. Not fair to public! —@realDonaldTrump

Never mind that pulling NBC's licence isn't possible because it does not, in fact, have a Federal Communications Commission licence to pull. Trump's very suggestion of using regulatory authority to silence journalism he dislikes raises constitutional concerns for Richard Painter, the former chief White House ethics lawyer for president George W. Bush.

"If he doesn't intend on respecting the freedom of the press, he cannot be president," Painter said. "He has authoritarian instincts. And if he doesn't want to uphold his oath under the Constitution, that's what impeachment is there for."

What Painter considers an authoritarian streak in the president may have been amplified on Wednesday, when Trump remarked from the Oval Office that he found it "frankly disgusting" that the press is "able to write whatever they want to write."

"People should look into it," Trump added, as Prime Minister Justin Trudeau sat in a chair beside him.

As constitutional experts grappled with what to make of a president who openly questions the press's freedom to report what it deems worthy, Republican senator Ben Sasse, of Nebraska, challenged Trump in a statement.

Trump said he found it 'frankly disgusting' that the press is 'able to write whatever they want to write' as he sat with Prime Minister Justin Trudeau in the Oval Office on Wednesday during NAFTA discussions. (Jonathan Ernst/Reuters)

"Words spoken by the president of the United States matter," Sasse said. "Are you tonight recanting the oath you took on January 20th to preserve, protect and defend the First Amendment?"

To Painter, that appeared to be the case.

"The point is, he's indicated that the First Amendment is 'disgusting' to him," Painter said.

'Disturbing rhetoric'

Whether Trump's remarks amount to a violation of the First Amendment depends on whether the words themselves are considered "government speech," protected as an expression of the government's viewpoint, said Yvonne Tew, an associate professor who teaches constitutional law at Georgetown University in D.C.

"If it consists of a threat by the government to regulate freedom of speech or to suppress the press" — which press-freedom advocates argue is the case — "then it would cross the lines into a dangerous zone."

Trump's Twitter posts would come under scrutiny, as former White House spokesperson Sean Spicer previously confirmed the president's tweets should be "considered official statements."

All presidents have at some point slammed their own press coverage. That's been fair game, said Gene Policinski, chief operating officer of the First Amendment Centre at the Newseum in Washington.

In that regard, Trump's criticism has been no different, though his rhetoric may have been deemed more toxic by pundits. Where the president goes too far, Policinski says, is when he brings up the idea of government retaliation against a news organization he objects to.

Presidents are not impeached for adopting extreme interpretations of the law. They're impeached for ignoring the law. - Jonathan Turley , constitutional law expert

"Certainly, if there were a serious attempt to silence the press, I think that would be a constitutional crisis."

The president's oath of office, recited at inauguration, includes a commitment to "preserve, protect and defend the Constitution"— and that means "every part of that Constitution," Policinski said.

Up for debate is whether the president's words themselves could be considered to violate the Constitution or breach the oath of office, which could be grounds for impeachment.

For his part, Jonathan Turley, a nationally recognized authority on constitutional law, believes words must be accompanied by "acting in an unconstitutional manner" for an impeachable offence to be considered, likely as a "high crime or misdemeanour."

So far, he notes, Trump has largely complied with constitutional rules and checks, such as when courts have ruled against him on his travel ban and attempts to deny funding to so-called sanctuary cities.

In this case, Turley said, the president is "advancing his interpretation" of FCC rules and the First Amendment.

"But presidents are not impeached for adopting extreme interpretations of the law. They're impeached for ignoring the law," he said.

There's often a disconnect between the president's statements and his actions. Still, Turley said that doesn't take away from Trump's "disturbing rhetoric."

"The idea that the government could start to pull licences by declaring organizations purveyors of 'fake news' is chilling," he said. "If that was not meant in jest, it seriously miscomprehends the constitutional and legal protections afforded to the media."

'Just hot air'

In a blog post, activist-journalist Trevor Timm, co-founder of the Freedom of the Press Foundation, argues that regardless of whether Trump can actually punish NBC by revoking its licence, his threat — even an empty one — may be enough to make a case actionable.

Timm references a 2015 case in the Seventh District in which a federal appeals court judge ruled, in effect, that it's not OK for a public official to threaten to use the power of the state even if that official lacks the ability or authority to mete out the punishment.

Rebecca Baker, president of the 7,000-member Society for Professional Journalists, a national organization, is keeping watch on whether the president does indeed use his government's powers to erode the constitutionally protected freedom of the press.

"Right now it's just his words — explosive, anger-driven words — but no actual actions taken against the media. No executive orders, no bills introduced to Congress, just hot air," she said. "But if this administration takes action to restrict the media, to change libel laws, to jail journalists, to do anything that's on that level, we will be on the front line."