Until now, Trump has been protected from prosecution largely because of a 1973 Justice Department memorandum that prohibits indicting a sitting president. But as Mueller’s report noted, that policy presupposes that a future administration can proceed with prosecution. The memo rests on a determination that a criminal charge would unfairly burden a president and hinder him from executing his duties. These concerns, of course, are no longer salient after the president leaves office.

This doesn’t mean that future prosecution is guaranteed. In fact, even if Trump were to leave office without taking any steps to immunize himself from prosecution, the next attorney general may take into account several factors that weigh against pursuing charges. The next Justice Department will have to review the evidence and determine whether prosecutors are likely to secure a conviction — an outcome that is by no means assured, especially given Trump’s profile and some of the constitutional defenses he might assert. Another consideration may be whether there are other jurisdictions — like the states — that are better suited to bring charges on the same evidence. The attorney general may also think about the impact on the nation of prosecuting a former president, and whether it would strengthen or weaken the public’s faith in the impartiality of the Justice Department. (History suggests that at least some of these considerations played into President Gerald Ford’s decision to pardon former president Richard Nixon.) The final decision is in the hands of the next attorney general, whom President-elect Joe Biden hasn’t yet named, but he or she would be remiss not to at least consider these factors, many of which lean in Trump’s favor.

All of this, however, would go out the window in the event of a self-pardon, because a self-pardon would effectively be a declaration by Trump that he is above the law, and that would be a direct challenge to the authority and legitimacy of the Justice Department. A self-pardon negates the very idea of prosecutorial discretion — the latitude to weigh all the factors above — and makes the president, as a possible future defendant, his own prosecutor, judge and jury.

It would also carry disastrous implications beyond Trump. Allowing a self-pardon would set a precedent whereby any future president could commit crimes — like soliciting or benefiting from foreign campaign assistance — to win office. Once there, he or she could abuse presidential powers for personal gain, engaging in obstruction of justice and perjury to thwart any investigations. Then the president could commit crimes up to the last day in office, like extortion and fraud, to remain in the White House illegally. If it works, and Congress doesn’t impeach and remove them, great! If not, they can pardon themselves and obtain a get-out-of-jail-free card on their way out. The presidency would effectively become a risk-free, crime-laundering opportunity, which is fundamentally at odds with a nation built on the rule of law.

But to challenge a self-pardon, the Justice Department has to charge the former president with a crime. This is because courts in the United States do not offer “advisory opinions” — they don’t answer legal questions in the abstract, only where there is an active “case or controversy” that affects a person directly. So there’s no way to know if the idea of a self-pardon is valid without testing it in court. The only person directly affected by the validity of a self-pardon would be Trump, and he would assert it as a defense only if the Justice Department charged him with a federal crime.