Civil and military relations have been “tested lately,” Mr. Biden argued, alluding to Mr. Trump’s efforts earlier this summer to use federal law enforcement to “dominate” demonstrators protesting police brutality amid a national outcry over racial injustice. Republicans in recent days have seized on renewed unrest in American cities to argue falsely that Mr. Biden wants to defund the police, an approach he opposes.

“I promise you, as president, I’ll never put you in the middle of politics, or personal vendettas,” Mr. Biden said. “I’ll never use the military as a prop or as a private militia to violate rights of fellow citizens. That’s not law and order. You don’t deserve that.”

Mr. Biden, who has been a firm supporter of peaceful protests, is expected to travel on Monday to condemn violence, and to note that chaos has unfolded on Mr. Trump’s watch, according to someone familiar with his plans. He is also expected to charge more broadly that the president is seeking to change the subject from the coronavirus and economic challenges the country faces. Details of his Monday plans weren’t immediately clear.