Harris spoke to NBC News just two weeks before Election Day. She said the Democrats "have the resources and the expertise" should Trump try to subvert the election.

"This is a person, Donald Trump, who tried to undo the -- a free and fair election, who still denies the will of the people, who incited a violent mob to attack the United States Capitol and some 140 law enforcement officers were attacked. Some -- were killed. This is a very serious matter," she said.

Trump has been criminally charged with trying to overturn the 2020 election, and refuses to admit he lost to President Joe Biden. After a failed legal effort to overturn the results, a mob of Trump supporters rioted at the U.S. Capitol on Jan. 6, 2021, attacking law enforcement in an effort to stop the certification of the race.

At Harris' rallies, some of her supporters chant "Lock him up," something Trump often said about his former Democratic rival, Hillary Clinton. Harris often replies: "The courts will take care of that. We'll take care of November."

At a campaign stop Tuesday, Biden said, "We've got to lock him up," but quickly added, "Lock him out, that's what I mean."

"No president has ever been like this guy," Biden said. "He's a genuine threat to our democracy."

While partisan battles over voting rules have long been part of presidential campaigns, election litigation has soared in recent years. With money pouring in for legal fights and the number of outside groups involved in election litigation proliferating, the disputes are not likely slow down anytime soon.