With three weeks left before Election Day, former President Donald J. Trump is pushing to the forefront of his campaign a menacing political threat: that he would use the power of the presidency to crush those who disagree with him.

In an interview on Sunday, Mr. Trump framed Democrats as a pernicious “enemy from within” that would cause chaos on Election Day that he speculated the National Guard might need to handle.

A day later, he closed his remarks to a crowd at what was billed as a town hall in Pennsylvania with a stark message about his political opponents.

“They are so bad and frankly, they’re evil,” Mr. Trump said. “They’re evil. What they’ve done, they’ve weaponized, they’ve weaponized our elections. They’ve done things that nobody thought was even possible.”

And on Tuesday, he once again refused to commit to a peaceful transfer of power when pressed by an interviewer at an economic forum in Chicago.

With early voting underway in key battlegrounds, the race for the White House is moving toward Election Day in an extraordinary and sobering fashion. Mr. Trump has long flirted with, if not openly endorsed, anti-democratic tendencies with his continued refusal to accept the results of the 2020 election, embrace of conspiracy theories of large-scale voter fraud and accusations that the justice system is being weaponized against him. He has praised leaders including President Vladimir V. Putin of Russia and Prime Minister Viktor Orban of Hungary for being authoritarian strongmen.