When Donald J. Trump told Hillary Clinton at Sunday’s presidential debate that if he were president, “you’d be in jail,” he was threatening more than just his opponent. He was suggesting that he would strip power from the institutions that normally enforce the law, investing it instead in himself.

Political scientists who study troubled democracies abroad say this is a tactic typical of elected leaders who pull down their systems from within: former President Hugo Chávez of Venezuela, President Robert Mugabe of Zimbabwe, the fascist leaders of 1930s Europe.

Today’s United States, unlike the countries in those cases, has strong institutions and norms that prevent any president from going that far, these experts stress. But Mr. Trump’s threat to jail his opponent for her deletion of thousands of emails sent from a private server while she was secretary of state, they warned in interviews on Monday, would chip away at the things that make American democracy so resilient.

Mr. Trump’s comment was “a threat to the rule of law, a threat to the stability of our institutions, a threat to basic agreements that are necessary for democracy to function,” said Adrienne LeBas, a political scientist at American University.