Still, there are limits to a pardon spree. As Attorney General William Barr testified during his confirmation process, a pardon granted in order to corrupt a judicial proceeding can amount to a criminal obstruction of justice. Mr. Trump’s pardons will be closely scrutinized for any purpose to thwart an investigation or pending prosecution threatening to him, a family member or close associate.

Mr. Trump has proclaimed “the absolute right to pardon myself.” While neither the Constitution nor judicial precedents overtly speak to the issue, the Justice Department declared in 1974 a self-pardon would “seem” to be disallowed “under the fundamental rule that no one may be a judge in his own case.” Scholars are torn on the matter. The issue, which would arise if after Mr. Trump leaves office the new administration indicts him for a crime for which he pardoned himself, can be settled only by the Supreme Court.

There is little that can be done at this point to stave off a probable wave of opportunistic pardons. But in light of what we already know about his pardon practices, Congress should enact two reforms to prevent future abuses.