“There is a check on a president who abuses the pardon power,” Brian Kalt, a law professor at Michigan State University and an expert on pardons, told me. “He gets voted out of office or he is impeached.” Although a president hasn’t yet been impeached for an unpopular pardon, Ford’s popularity did suffer after he pardoned Nixon, and that may have cost him the 1976 election. Presidents George H.W. Bush and Bill Clinton also granted risky pardons — Bush functionally ended the Iran-Contra investigation, in which he was implicated, by pardoning six former Reagan administration officials, and Clinton pardoned his half-brother. But both Bush and Clinton granted these pardons in the waning days of their presidencies, when the political consequences weren’t as severe.

But generally, even though past pardons have been criticized and even investigated, the legitimacy of a president’s pardon hasn’t been questioned in the way that Trump’s pardon of Arpaio is being debated. Trump’s actions and rhetoric have, however, exposed a loophole in the pardon power: Even if a pardon is arguably an abuse of presidential power, there’s currently no clear way to undo the pardon itself. So now the question before the courts is whether Trump’s actions are extreme enough to warrant blowing up the constitutional status quo. Advocates of pardon restrictions say that their arguments really aren’t that radical — they just think pardons shouldn’t override the rest of the constitutional system. What remains to be seen is whether the courts feel the same way — or whether they’ll tell Trump’s critics to take their grievances to Congress.