The Democratic majority on the House Judiciary Committee sent letters to 81 people and organizations on Monday, seeking more information about subjects related to President Trump, his private business, his transition team, his inaugural committee, his foundation and his presidency. The move, coupled with testimony last week by Trump’s former personal attorney Michael Cohen, marked the beginning of a robust new effort by Democrats to dig into every murky aspect of Trump’s recent life.

That broad request for information almost certainly will trigger a broad effort to push back from people still loyal to Trump and his presidency. Many of the letters’ recipients will ignore the initial requests for information, forcing Democrats to issue subpoenas to gather information important to their investigations. Some, either in response to the initial letters or subsequent subpoenas, will claim that their documents are protected by “executive privilege.”

This may be an unfamiliar term to some people, or perhaps it’s a term that’s understood as generally meaning that any conversations that Trump, the chief executive, thinks should be kept private can legally be kept private. That’s not what it means or what it allows. Given how likely it is that this will come up as the Democrats press forward, it’s worth explaining exactly what executive privilege does and doesn’t allow Trump to do — for now.

“Executive privilege is not in the Constitution,” said Steven Schwinn, a law professor at John Marshall Law School in Chicago, when we spoke by phone Wednesday. “It’s a creation of the Supreme Court in the United States v. Nixon case, growing out of the Watergate prosecution.”

If that case rings a bell, it should. It’s been referred to frequently in recent months, given the overlap between the events that led to the resignation of President Richard Nixon and the current effort by Trump to push back against the investigation being conducted by special counsel Robert S. Mueller III. 1974′s U.S. v. Nixon established important precedent on a number of things related to the power of the presidency — including the idea of executive privilege.

“What the Court said in that case is that there is such a thing as executive privilege that protects high-level decision-making in the executive branch and, in particular, presidential communication with high-level advisers,” Schwinn explained. “But it’s not an absolute privilege; it’s a qualified privilege. And so what the Court said in United States v. Nixon is that the courts, when evaluating a claim of executive privilege, should balance the reason for the invocation of the privilege against the need for the requested information.”

What Nixon’s attorneys had argued was a broad right to protect internal White House communications. The Supreme Court accepted that there was some need to protect such communications, especially in the case of national security or other state secrets. It also determined that the privilege itself was dependent upon circumstance. It’s subjective, in other words, relying on a judge to determine whether the executive’s need to protect the information is outweighed by the need of the other party to have access to it.

If a president were trying to keep a criminal court from finding out that he’d discussed murdering someone with his chief of staff, that’s a different situation than if Congress is subpoenaing a president for war plans. Who’s asking, what they’re asking for and what the information actually says are all weighed in the consideration of the court.

So how does it work in practice? In a traditional use, Schwinn explained, someone who had worked in the White House and was being subpoenaed would identify communications that fell under the scope of the subpoena and which involved the president. They would then ask the White House if it wanted to exert privilege and, if so, inform the requesting party. Again: It’s only the White House that can claim that privilege, just as only an attorney’s client can waive attorney-client privilege. But notice the difference: Executive privilege doesn’t automatically exist in the way that attorney-client privilege does.

Under Trump, invocations of privilege have gotten a bit looser than the traditional scenario Schwinn offered. Members of the administration have declined to answer questions under oath while waiting for the White House to decide if it plans to exert privilege. At another point, former adviser Stephen K. Bannon offered a sweeping assertion of privilege at the White House’s direction, to the point that one Democrat called it “laughable.”

White Houses, in an eternal and intentional power struggle with Congress and the courts, will often push the boundaries of privilege. Schwinn thinks Trump is pushing further than most.

“Either party is going to invoke executive privilege in a reasonable way, but one that is nevertheless aggressive,” he said. “In my view, what’s unusual about what President Trump is doing, and some of the folks associated with him, is that they’re just invoking it like it is a trump card and like they can just pull it out of the back pocket and put a halt to anything that Congress has to request. That, in my view, is not only aggressive, but it is just entirely unreasonable.”

Bannon’s testimony came when Republicans ran the House Intelligence Committee. Democrats, now in the majority, would likely have pressed harder on his assertions of privilege.

Past fights along these lines have often been resolved between Congress and the White House, Schwinn said. That may break down under Trump.

“My sense is that the Trump administration is far less willing to accommodate on these questions than other administrations have been,” he said. “Where the courts have largely stayed out of this and allowed Congress and the White House to kind of work out some sort of deal to get the information — but to do it in a way that provides protection — I’m not sure the Trump administration is going to be that accommodating. They might just go to court. And given the composition of the Supreme Court, changes that we’ve seen certainly since 1974, that might be a reasonable bet on the administration’s part.”

After all, a court might end up having to answer a number of questions that have been fairly nebulous since executive privilege was created. Does it apply only to senior administration officials, or could it extend to people outside government with whom Trump has had conversations? Does it cover senior staffers talking to one another and, if so, to what extent? How can or should it be invoked?

You can probably see one minefield where this is headed. While we have know-it-when-we-see-it examples of where privilege would apply — e.g., Trump talking to his CIA head about an imminent terrorist attack — how it is used will come down to how a court thinks it can be used. To Schwinn’s point, the current Supreme Court might be more inclined to see Trump’s point of view on a particular issue than the court led by former chief justice Warren Burger was to see Nixon’s.

What we might expect, then, is this: Several of the 81 identified individuals on the House Judiciary Committee list will oppose turning over documents. Some will claim executive privilege in doing so, perhaps even some who never worked in the White House. Democrats will push back on those claims — which could be numerous — and the White House will probably dig in on most of them.

The Democrats will then have a choice: Give in or take it to the courts, potentially establishing new rules governing the power balance between themselves and the White House. Just as claims of executive privilege themselves are weighed on the balance between what’s requested and who’s requesting it, the Democrats will then have to decide: Is it worth it?