Donald Trump speaks to the Republican Hindu Coalition in Edison, N.J., on Saturday. (Evan Vucci/Associated Press)

WHAT HAS allowed the United States to last for so long as a democracy, when so many other countries have failed? There are many factors, but none is more fundamental than this: When we hold elections, the losing party acknowledges the legitimacy of the winner, and the winner allows the loser to survive to fight another day.

Now, for the first time in modern history, a major-party candidate rejects both sides of that equation. If he loses, Donald Trump says, it will be due to cheating that makes the result illegitimate. If he wins, he will imprison his defeated opponent.

Many Americans may not have given much thought to what a breathtaking departure this represents, because until now we have had the luxury of never having to think about such things. We have been able to take for granted the quadrennial peaceful transition of power. We watch from a distance when political parties in one foreign country or another take up arms after losing an election. We look, as at something that could never happen here, when a foreign leader sends an opponent to jail or into exile. This can happen in Zimbabwe, we think, or Russia, or Cambodia, but not here. Not in the United States.

The Republican nominee is saying that he will make it happen here. He tells Hillary Clinton, the Democratic nominee, that if he were president, “you’d be in jail.” He nods approvingly and chimes in when his crowds chant, “Lock her up.” He warns that a vast if fuzzily defined conspiracy of global bankers, media companies and election officials is gearing up to steal the election. “The election is rigged,” he says. “It’s rigged like you have never seen before. They’re rigging the system.”

We have endorsed Ms. Clinton for president, contending that she is well qualified, well prepared and likely to do a good job. But to voters who disagree — who have never voted for a Democrat, say, or who question our assessment of her qualifications — we would argue that Mr. Trump’s challenge to the very core of our democracy nonetheless provides strong reason to vote for her.

At a campaign rally in Portsmouth, N.H., on Oct 15, Republican presidential candidate Donald Trump said the election was being rigged by "corrupt media" and added that the media is pushing "false allegations" and "lies" regarding sex assault claims against him. (The Washington Post)

You may disagree with Ms. Clinton about Obamacare, Russia policy or Planned Parenthood. She may, as president, take actions that deeply upset you. But you can be certain that if Republicans take issue with her, she will not order them jailed.

With Mr. Trump, if the candidate himself is to be believed, there is no such certainty. A voter’s first obligation should be to preserve the republic which has been, for so long, the envy of the world.