George Washington’s legacy is restraint, particularly on presidential power. That legacy is threatened like never before.

Last night, Donald Trump told Hillary Clinton he plans to put her in jail if he’s elected President of the United States.

Dictators threaten to arrest political rivals. American presidential candidates never have. One of the central features of our democracy, since the Founding Fathers, is that we do not use the criminal justice system against political opponents. We separate the two realms as much as possible, to protect political freedom.

Mr. Trump has repeatedly said he admires the methods of dictators, rather than the restraint of George Washington, which has guided American politics for more than two centuries. Last night was another chilling example. Mr. Trump does not understand democracy, and his election could lead to a quasi-democratic regime like Putin’s in Russia, which he so admires, or Erdoğan’s in Turkey — or to civil war.