The most crucial calculation for a self-pardoning president is when the pardon takes place. Suppose that Trump decides to fire Special Counsel Robert Mueller and also pardon himself and his associates for any federal crimes they may have already committed. Everybody’s off the hook.

But there’s a catch: One possible charge against Trump is that he has conspired to obstruct justice—and continues to do so. As of the date of the pardon, past acts of conspiracy could no longer be prosecuted. But if Trump blocked investigations after the pardon, he would be committing fresh acts of obstruction—which the pardon could not have covered. According to Samuel W. Buell, a former federal prosecutor who teaches a seminar at Duke University called “The Presidency and Criminal Investigations,” if the president were prosecuted for the post-pardon acts, his pre-pardon conduct could be used against him in court—for instance, as evidence of “the background and purpose” of any conspiracy he was charged with.

Moreover, the pardon itself may be a criminal act under federal law. “Lawful acts can constitute obstruction of justice when done with a bad motive,” Buell says. “I don’t see why a pardon would be any different, if it’s done for the purpose of keeping the president from being held to account.”