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.

Nicholas Burns, a former undersecretary of state under George W. Bush, said Mr. Trump’s refusal to say he would respect the outcome of the presidential race was a “colossal mistake” that could damage American prestige abroad.

“What many if not most foreigners admire about us, about the United States, is the durability of our democracy and the fact that we alternate power,” Mr. Burns said. “It’s how we are fundamentally different from Russia and China, and it gives us an enormous advantage.”

Mr. Burns, who has endorsed Mrs. Clinton for president, called Mr. Trump’s remarks a flagrant violation of American democratic traditions. “I don’t think we’ve had a serious national leader say that since the Confederate leaders of 1860 who refused to accept the election of Lincoln,” he said.

William M. Daley, a former White House chief of staff who was the chairman of Al Gore’s campaign during the 2000 standoff in Florida, said Mr. Trump seemed indifferent to the possibility that his words might weaken trust in the American government, at home and overseas.

Mr. Daley said the Gore team had been cautious not to say anything during the Florida recount that might cripple the next president’s legitimacy, to the point that other Democrats criticized them for being overly cautious. Mr. Trump, he said, was taking the opposite approach.

“He really has no appreciation for our history, which most of the world looks at with great admiration, as opposed to some banana republic,” Mr. Daley said. On election night, Mr. Daley said, “he could be tweeting at 3 in the morning and trying to undercut the new administration coming in.”

The United States has endured contested election results, most recently in 2000, but historians and political experts could recall no case of a presidential nominee attacking the electoral process, with no apparent instigation or factual basis, with weeks until Election Day, as Mr. Trump is doing now.

It is unclear whether Mr. Trump has a concrete plan to contest the results of the election if he loses. There is no law that forces a losing candidate to concede defeat — only a bipartisan tradition of comity. But fighting the apparent outcome of a presidential race could require elaborate litigation across numerous states, with virtually no hope of success without hard evidence of extensive fraud.

Mr. Trump has presented no such evidence, instead offering sweeping denunciations of the overall political process, the news media and the judicial system. His language has seemed to conjure images from the developing world and unsteady new democracies — countries like Myanmar and Nigeria, where governments have overturned election results by fiat, and political turbulence has given rise to outbreaks of violence.

In the United States, modern presidential standoffs have been tense but restrained: Mr. Gore mounted an aggressive legal challenge to election results in Florida, but he conceded swiftly once the Supreme Court ruled against him. When John F. Kennedy narrowly defeated Richard M. Nixon in 1960, Nixon publicly renounced his claim to the presidency even as his supporters pressed forward with recounts on the state level.

Only a few presidential elections, with painfully inconclusive results, have yielded more turbulent challenges after Election Day. Two 19th-century presidents who failed to win the popular vote, John Quincy Adams and Rutherford B. Hayes, entered office battered by opponents who said they had won through corrupt means. Neither man served a second term.

In American electoral history, there is only one instance of the losing side in an election simply declining to abide by the outcome, without any substantive legal objection or mathematical uncertainty around the results. That was in 1860, when Abraham Lincoln’s election prompted the secession of Southern states that refused to tolerate his opposition to slavery.

Despite the heated passions of Mr. Trump’s political following, he appears unlikely to steer the country into civil war. Still, he has unnerved many American political leaders through what they view as his sheer recklessness.

Benjamin L. Ginsberg, a prominent Republican election lawyer who represented Mr. Bush against Mr. Gore in their 2000 impasse, said Mr. Trump was taking the country into unknown territory.

“When people see a close election and exert their rights to the fullest is what we have always considered within bounds,” Mr. Ginsberg said. “But saying somehow that our elections are not going to be accurate and do not adequately convey the will of the majority is different from what we have ever seen before.”

Mr. Trump’s campaign has long attracted unusual interest, and in some cases extraordinary alarm, in foreign countries. Several foreign leaders have intervened in the American election to attack his candidacy in strikingly blunt terms; Manuel Valls, the prime minister of France, declared flatly this month that his country supported Mrs. Clinton.

Mr. Trump has also been endorsed by an array of hard-right leaders across Europe, and he has campaigned alongside Nigel Farage, the nationalist politician who led the charge for Britain to leave the European Union.

Mr. Trump’s attack on the American electoral system quickly stirred reaction from activists preoccupied with matters of international democracy and freedom. On Facebook, Shen Tong, a former leader of the Chinese pro-democracy movement who organized protests in Tiananmen Square, wrote that Mr. Trump’s comments had been jarring for anyone who expected an easy “national election and peaceful transition of power in the USA every 4 years.”

Mr. Trump’s “suspense on his acceptance of the election result reminds me how unique this constitutional democracy has been,” Mr. Shen wrote, “and how much we’ve taken it for granted.”