Donald Trump’s presidency has been defined by a central theme: Trump’s belief that ordinary rules and laws do not apply to him.

Trump has made clear that he believes it is up to his personal discretion to order torture – even though torture is illegal under all circumstances. In ordering a military strike against Syria in April, Trump brushed aside constitutional requirements that Congress approve such action unless the U.S. faces imminent attack. And he has defended his presidency by falsely claiming that the president is incapable of having conflicts of interest.

I have argued in the past that Presidents George W. Bush and Barack Obama showed there is reason to be concerned about post-9/11 presidents testing the legal limits of their power. The stakes are even higher now with Trump. He has demonstrated authoritarian tendencies and contempt for the rule of law that goes beyond anything Bush or Obama did.

The issue may be coming to a head with investigations into Russian interference in the 2016 election and possible obstruction of justice. As the nation has watched witnesses appear before congressional committees and read Trump’s tweets about Department of Justice officials, the key question to ask now is whether Trump will refuse to let any investigation continue. If he does so successfully, Trump will effectively place himself beyond the reach of the law.

The rule of law

The various ongoing investigations are all, in theory, governed by legal rules. Special Counsel Robert Mueller’s task is to speak to witnesses, review documents, gather evidence and decide whether there is any basis for prosecution under federal law. Congressional committees, meanwhile, hear from witnesses who testify under penalty of perjury if they lie under oath.

But such legal rules are not self-enforcing. When the rules are violated or flouted, someone has to act in order to give them force and meaning.

Attorney General Jeff Sessions’ recent testimony before the Senate Intelligence Committee is a case in point. Sessions refused to answer a number of questions about communications he’d had with the president. By itself, that is not extraordinary. If the communications were protected by executive privilege or involved classified information involving national security matters, there may have been a legitimate basis for Sessions to decline to answer senators’ questions. After all, the Supreme Court has recognized that the Constitution implicitly allows the president to invoke executive privilege in some circumstances in order to protect the confidentiality of discussions with close advisers in the executive branch.

But Sessions, the top lawyer for the U.S. government, did not point to any legal grounds for his refusal to respond. He simply said he could not speak about private conversations he’d had with the president, and that he was protecting Trump’s ability to claim executive privilege, if he later decided to do so.

Sessions was not the first. A week earlier, Director of National Intelligence Dan Coats similarly declined to answer questions involving conversations he’d had with the president. Like Sessions, Coats did not invoke privilege, conceding that he wasn’t sure there was any legal basis he could rely on.

As Sen. Martin Heinrich noted during the hearing, that’s not the way executive privilege is supposed to work. If the administration wants to invoke the privilege, it must do so expressly. In that case, the matter would be worked out either in negotiations between the executive and legislative branches or (less frequently) through review by the federal courts.

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The most famous example of a court weighing in on executive privilege was the Supreme Court’s 1974 decision in U.S. v. Nixon. President Richard Nixon’s administration refused to hand over Oval Office tapes, claiming recorded conversations were protected by executive privilege, as defined by the president. The court rejected this view, observing that constitutional separation of powers depends on checks and balances that prevent any one branch from self-policing. The court found, in this case, the need for checks on power outweighed the executive branch’s interest in keeping discussions confidential. With the specter of impeachment looming over him, Nixon was forced to hand over the tapes. He resigned from office a few weeks later.

At the close of Sessions’ testimony, Sen. Richard Burr instructed Sessions to “work with the White House to see if there are any areas of questions that they feel comfortable with you answering…” That’s not good enough: If the legislative branch is to enforce the rule of law, witnesses must be compelled to answer legitimate questions under oath.

Will Congress act?

Special Counsel Mueller may be investigating the president to determine whether his actions amount to an obstruction of justice. Trump has already fired former FBI Director James Comey, and there is speculation that he might also fire Special Counsel Mueller in an effort to bring the investigation to a close. Sen. Ron Wyden has warned that, if Trump fires Mueller, it would be an attack on the rule of law itself. The onus would fall squarely on Congress to either initiate impeachment proceedings or else acquiesce in a presidential power grab.

As Sen. Heinrich noted, when witnesses refuse to answer questions but fail to provide any sufficient legal reason for doing so, they are obstructing investigation – preventing Congress from carrying out its inquiry. If other senators agreed, they could vote to cite the witness(es) for contempt, which could lead to criminal prosecution.

Congress could also threaten to hold up Trump’s nominations to key positions such as federal court judges, or refuse to move on the administration’s legislative priorities like tax cuts for high earners (it took some similar actions in response to Nixon). Congress could even begin impeachment proceedings if it decided presidential misconduct rose to the constitutional level of “high crimes and misdemeanors” – for instance, if Mueller’s investigation concluded that there is evidence to support this conclusion.

Of course, since Republicans are members of the same party as the president, none of this is likely – yet. But if Trump administration officials continue to make investigation difficult, and if Trump escalates an already tense situation by continuing to question Mueller’s legitimacy or even by firing the special counsel, Republicans may face a crucial test on behalf of American constitutional democracy.