During former U.S. Ambassador to Ukraine Marie Yovanovitch's public testimony last week, President Trump took to Twitter to criticize her. The tweet has since prompted officials, including House Intelligence Committee chairman Adam Schiff, to accuse the president of witness intimidation. WGBH Morning Edition Host Joe Mathieu spoke with Northeastern University law professor and WGBH News legal analyst Daniel Medwed about whether President Trump did, in fact, engage in witness intimidation, and what that means. The transcript below has been edited for clarity.

Joe Mathieu: Is this witness intimidation from the president, in this case? How do you define it?

Daniel Medwed: Well, on the one hand, the effect was intimidating, as Ambassador Yovanovitch indicated. On the other hand, the key issue is the intent. What was President Trump's intent? Was his goal to intimidate a witness into testifying a particular way? So here's how it works: Witness intimidation is a subspecies of the broader category of witness tampering. The federal statute, which is 18 U.S. code section 1512, basically makes it a crime to improperly influence a witness to testify a particular way. And its broad language encompasses not just classic intimidation tactics, like the threat or use of force, but also broad behavior like corruptly persuading someone to testify in a certain direction.

Mathieu: There was another tweet just a couple of days ago from President Trump about one of the individuals testifying this morning: Jennifer Williams, the aide to Vice President Mike Pence. The president writes, "Tell Jennifer Williams, whoever that is, to read BOTH transcripts of the presidential calls, & see the just released statement from Ukraine ... work out a better presidential attack!" Would that fall under this same guise you're talking about?

Medwed: Quite possibly. Again, the effect would seem to be an intimidating one, a tampering one. But the key issue: What was his intent? Is his intent to pervert the testimony of the witness? Or maybe just to speak his mind and exercise his First Amendment rights? I imagine that's a defense he would raise.

Mathieu: So if someone's convicted of witness tampering, what does that mean? What's the punishment?

Medwed: The range is incredibly wide. At the high end, under the federal statute, if you use force or the threat of force, you're looking at a maximum sentence of 30 years. At the bottom end, if you only engage in intentional harassment, you're looking at three years. Many other types of behavior are subject to 20 years.

Mathieu: Daniel, how often are people prosecuted for witness tampering or intimidation? Is this typically reserved for organized crime, or does it come up in everyday situations?

Medwed: It clearly does apply to organized crime situations, though notably those cases are difficult to prosecute because of the culture of silence that often enshrouds organized crime activities and the genuine fear of reprisals or retributions for witnesses. So they might not come forward. But we have seen a very recent high-profile case outside the organized crime context involving witness tampering: Roger Stone. Trump's ally and friend was just convicted a few days ago of seven criminal charges including witness tampering. That charge related to efforts by Stone to lean on a former confidant, Randy Credico, to get him to testify or not testify a particular way about Julian Assange and WikiLeaks. Stone allegedly said, "Stonewall it," "Plead the fifth" or "Pull a Frank Pentangelli," [which is] a reference to a character from The Godfather movie series who notoriously lied in front of Congress. And to add insult to injury, the evidence revealed that Stone even threatened Credico's therapy dog, Bianca.

Mathieu: So how do you defend against a charge of witness tampering? What are we going to see next year?

Medwed: Well, I think as I alluded to before, the first argument is that the prosecution has to prove intent beyond a reasonable doubt. And a lot of defendants would say, "It wasn't my intent to improperly influence, intimidate or tamper with this witness. I was just speaking my mind." Second, there's an affirmative defense that's embedded in the federal witness tampering statute that says even if you pressure a witness to testify a particular way, if you're trying to urge them to tell the truth, that's a safe harbor.