President Trump has claimed executive privilege over Special Counsel Robert Mueller's final report to prevent Congress from accessing the unredacted document. But what exactly is executive privilege? Morning Edition Host Joe Mathieu spoke with Northeastern law professor and WGBH Legal Analyst Daniel Medwed to discuss the extent of executive privilege and the arguments against it. The transcript below has been edited for clarity.

Joe Mathieu: So let's start with the basics. How should we actually define executive privilege for the sake of this conversation?

Daniel Medwed: In a general sense, it just refers to a claim by a member of the executive branch that certain information should be shielded from disclosure to the other branches — to the courts and to Congress — to keep the government running smoothly. In a more practical sense, presidents since, I believe, Thomas Jefferson have cited variations of this privilege to justify withholding information. And things reached a head in 1974, the height of the Watergate era, when there was a grand jury subpoena for President Nixon to turn over a lot of information related to the investigation against him. He refused to comply, citing executive privilege. The case went up to the U.S. Supreme Court, which for the first time recognized the constitutional parameters of this privilege in a 9-0 decision. So presidents may raise it with respect to certain information.

Mathieu: So as we test the boundaries of executive privilege under President Trump, what are the limits of this concept, if there are any?

Medwed: Well, there are limits. I'll admit that the case law is pretty sparse, but here's what we know. Confidential communications between a president and his adviser lie at the core of executive privilege. You want to ensure candor in the White House [and] you want a free flow of ideas. There's a fear that there could be a chilling effect on those conversations if people thought that the communications could later be disclosed. In addition, executive branch officials have asked for a much broader, deliberative process privilege that would apply outside the White House and would apply to any sort of executive agency deliberation. There have been claims about sensitive law enforcement information [and] national security information potentially being covered. So those are the basic buckets of information.

Mathieu: The Department of Justice has indicated that Trump was making a claim of protective executive privilege with regard to this unredacted Mueller Report and the subpoenaed materials that came with it. What is protective executive privilege?

Medwed: Well, protective executive privilege differs from a more formal assertion of privilege in the following way: a protective claim is largely preemptive. You're basically preserving your right to later claim privilege and you want to take time to review all of the materials — in this case, maybe millions of pages and all of this information — take time to mull it over preemptively, and then possibly later raise a formal assertion of privilege. It's basically a two-step process. But the practical effect of this is that it's going to be a stall tactic. If in fact they're allowed to review all this material, it's going to take months for this all to play out.

Mathieu: Maybe until past the next election?

Medwed: Quite possibly.

Mathieu: Most [people] assume this goes to the courts. Is there any way to defeat these claims of privilege in that case?

Medwed: Yes, absolutely. I see three potential avenues for defeating a claim of privilege. First, Congress could pick a battle with a particular claim of privilege and say this item isn't privileged. Maybe that one is, but this one's not. Second, that 1974 case by the U.S. Supreme Court with respect to President Nixon, it acknowledged an executive privilege, but it didn't create an absolute privilege. It's called a qualified privilege, which means the privilege can be overridden if Congress presents compelling reasons for disclosure. So here Congress presumably is going to say the integrity of the country is at stake. We need to get to the bottom of any potential wrongdoing and they would hope that that could override the privilege.

Finally, there's a potential claim of what's called waiver. A lot of this information is already out in the public zeitgeist. For instance, Don McGahn has said things [and] he's even been interviewed, apparently, by Special Counsel Robert Mueller. To what extent does that waive the privilege? After all, something is no longer confidential if it's been disclosed through some other means. So there's probably going to be a big battle over whether or not those disclosures equal just a partial waiver of the privilege or a more comprehensive one.