Donald J. Trump’s suggestions that he might reject the results of the American election as illegitimate have unnerved scholars on democratic decline, who say his language echoes that of dictators who seize power by force and firebrand populists who weaken democracy for personal gain.

“To a political scientist who studies authoritarianism, it’s a shock,” said Steven Levitsky, a professor at Harvard. “This is the stuff that we see in Russia and Venezuela and Azerbaijan and Malawi and Bangladesh, and that we don’t see in stable democracies anywhere.”

Throughout October, Mr. Trump has claimed, without evidence, that the vote will be “rigged” and “taken away from us.” At the final presidential debate, he refused to say he would accept the election’s outcome, and later joked at a rally that he would accept the results “if I win.”

In weak democracies around the world, scholars warned Friday, political leaders have used the same language to erode popular faith in democracy — often intending to incite violence that will serve their political aims, and sometimes to undo democracy entirely.