It is a scene reminiscent of other countries and other times: An angry candidate defies the will of the voters and hurls venom at the democratic process. Threats of jail are issued against political opponents. There is even loose talk of armed insurrection.

With his assault on the legitimacy of the presidential election, Donald J. Trump threatens to touch off a humiliating spectacle unseen in the United States since the country became a global power.

Diplomats and elected officials in both parties fear that Mr. Trump, if he loses, will inflict grave trauma on the electorate and severely undermine the international reputation of an American political system known for revering the peaceful transfer of power.

Though he trails Hillary Clinton by a wide margin in most polls and has been abandoned by much of his own party, Mr. Trump still commands a powerful bully pulpit that he may use to amplify his unsupported claims that American democracy is a fraudulent system.