Will President Trump sit down for a one-on-one interview with special counsel Robert Mueller? It might be more likely after the president’s lead lawyer on the Russia investigation resigned last week. The lawyer, John Dowd, had reportedly advised the president against such a move, and now he’s out.

If Trump does talk to Mueller, there’s a possibility that he could invoke executive privilege to try to avoid answering some of the special counsel’s questions. That would be the first formal invocation of executive privilege in the investigation into Russian interference in the 2016 election, although legal experts say Trump is already expanding the power by instructing or allowing aides to refuse to answer questions in congressional testimony in case he wants to claim it later. The power of executive privilege isn’t unlimited. But because the courts and Congress have never established firm boundaries around it, any invocation of executive privilege in this context, whether formal or informal, could lead to a legal showdown in the courts if Congress or Mueller decided to challenge him — and it could also have serious political ramifications.

Executive privilege refers to a power that the chief executive can claim to withhold information from Congress, the judicial system and the public. You won’t find the term in the Constitution, but nearly every president starting with George Washington has exerted some version of this authority to keep information out of the hands of Congress. That has sometimes been because of national security concerns, but it has also been justified as protecting the privacy of White House deliberations. It’s impossible to have candid conversations with aides, many presidents have reasoned, if the details could be requested by Congress at any time.

Executive privilege didn’t gain its official name, though, until President Dwight Eisenhower and his administration began using it in the 1950s. And it took another 20 years for the Supreme Court to formally recognize executive privilege as a presidential power. In a decision about the Watergate tapes, the court clarified that the president can invoke executive privilege — but said that it didn’t apply to all claims.

The justices in that ruling, though, didn’t do much else to illuminate what executive privilege meant. This has led to a complicated dance between presidents, Congress and the courts as executive privilege claims are tested on a case-by-case basis. Sometimes the claims have gone unchallenged. Other times, Congress or special prosecutors have disputed them, and several have been thrown out in court after judges determined that they were more in service of the president’s own goals than the public interest.

This means that there’s a lot of uncertainty about what Trump can actually decline to reveal by claiming executive privilege. Ultimately, we’ll only know if an executive privilege claim is kosher — legally and politically — if Congress or Mueller contests it. But political considerations would likely play a role in those decisions: A challenge from Congress — like a contempt citation or civil action — would need majority support in a committee or the full House or Senate, and Republicans have so far shown little appetite for confronting the leader of their party in this way. Meanwhile, Mueller may not see a fight over executive privilege as strategically wise.

An invocation of executive privilege by Trump to avoid giving testimony in the Mueller investigation could backfire if it were challenged, though. According to experts, courts generally haven’t been receptive to broad executive privilege claims, especially when they could hinder a criminal investigation like Mueller’s. And in the past, excessive use of executive privilege has been floated as grounds for impeachment.

Sweeping claims can be problematic

Eisenhower began invoking what he labeled executive privilege to prevent Cabinet members and other senior advisers from being questioned in Congress during the controversial Army-McCarthy hearings, when Sen. Joe McCarthy picked a fight with the Army over their supposedly lax security. His actions weren’t disputed because of McCarthy’s unpopularity, and Eisenhower ultimately claimed executive privilege more than 40 times throughout his presidency.

Since then, though, presidents have pushed the concept’s limits in episodes that have ended up in high-profile legal confrontations over potential abuse of the power. The presidents immediately following Richard Nixon used executive privilege sparingly, perhaps because it was perceived as politically tainted, but starting with Bill Clinton, subsequent chief executives used the power more aggressively and received a significant amount of pushback from Congress and special prosecutors. Often, these confrontations resulted in a compromise, but in a handful of cases, the president’s actions were challenged in the courts for pushing the boundaries of the privilege too far.

“Executive privilege is there in service of the public interest — it’s not there to save the president from embarrassment or revealing politically inconvenient information,” said Mark Rozell, a professor of public policy at George Mason University and a leading expert on executive privilege.

Invocations of executive privilege, from Kennedy to Obama Presidency Number of invocations Clinton 14 W. Bush 6 Nixon 6 Reagan 3 Johnson* 3 Kennedy 2 Obama 1 H.W. Bush 1 Ford 1 Carter 1 *Under President Johnson, officials withheld information about executive actions from Congress, although Johnson didn’t formally invoke executive privilege. Source: Congressional Research Service

Although not all unsuccessful assertions of executive privilege involved court action, previous presidential successes and failures provide some clues about what kinds of executive privilege claims are most likely to run afoul of the courts.

In the past, judges have swatted down presidents when they’ve tried to make claims that were too broad. In 2008, a judge rebuked President George W. Bush for making the “unprecedented” claim that senior White House officials were beyond the reach of subpoena power in an investigation.

The problem, according to legal experts, was that presidents have typically been required to have a specific reason for invoking executive privilege rather than seeking to exempt an entire subject or cohort. This could be a problem for Trump as well, because several Trump aides have refused to answer whole categories of questions in testimony before Congress, saying that they were declining to provide information in case the president decides to claim executive privilege on these topics later.

Courts have also looked askance at presidents who have tried to withhold records in situations in which some of the information was already publicly available — like President Barack Obama’s failed attempts to keep documents about the Bureau of Alcohol, Tobacco, Firearms and Explosives’ botched “Fast and Furious” gun-trafficking investigation out of the hands of Congress.

The leakiness of the Trump White House could pose problems on this front if Trump invokes executive privilege to avoid providing information that has already made its way into the press. “If some of the evidence is already out there in a reporter’s notebook, it’s much harder to say that this information will really be damaging for the executive branch if it’s released to Mueller or Congress,” said Andrew Wright, who is a professor at Savannah Law School and worked as a White House lawyer under Obama.

Criminal cases have more weight

Judges have been particularly dubious of executive privilege claims when they appear to interfere with criminal investigations involving the president. This was first tested in 1974, when Nixon tried and failed to use executive privilege to withhold incriminating audio tapes in the Watergate investigation. The Supreme Court acknowledged the existence of some sort of executive privilege but ruled that there are times when it can be overridden, particularly when the president is trying to suppress evidence that’s relevant to a criminal investigation.

Nearly 25 years later, a judge reinforced this principle when she rebuffed Clinton’s attempt to use executive privilege to prevent several aides from testifying about internal White House discussions related to the special prosecutor investigation of his affair with Monica Lewinsky.

For Trump, this means that trying to use executive privilege to avoid participating in Mueller’s probe could be extremely difficult. According to Rozell: “In cases where information is absolutely needed to properly investigate a crime, the executive privilege weakens and disappears.”

Executive privilege is still political

Even if a battle over executive privilege doesn’t make it to the courts, executive privilege claims that appear to help the president avoid releasing information that is personally damaging could hurt Trump politically and thus pave the way for Congress or Mueller to challenge him when they might otherwise be reluctant to do so.

“To the extent that executive privilege is still associated with Nixon, it can look suspicious to invoke it,” said Heidi Kitrosser, a law professor at the University of Minnesota.

Claiming executive privilege could also create a weapon for impeachment. In the original articles of impeachment offered against Clinton by the House Judiciary Committee, the president’s use of executive privilege was called frivolous and corrupt (the charge was later dropped). “At one point, Clinton’s attorney made the argument that divulging information harms national security because anything that embarrasses the president weakens the nation’s standing internationally,” Rozell said. “It was an unbelievable stretch.”

Ultimately such political openings might be crucial for a challenge to Trump’s use of executive privilege. But legal experts say that’s not the way the system is supposed to work. “It’s a powerful privilege, but it’s not a get-out-of-jail-free card,” said Steven Schwinn, a John Marshall Law School professor. “When an executive invokes executive privilege, it’s a very serious and weighty thing, and we need to be willing to challenge possible abuses of the power.”