In a visit with handpicked border officials and Republican allies in Yuma, Ariz., Mr. Trump sought to revive the issue at the heart of his 2016 victory: his portrayal of immigrants as a threat to the economic and personal security of Americans, and his promise to close the United States off from much of the world.

“My administration has done more than any administration in history to secure our southern border,” Mr. Trump boasted, citing the completion of about 220 miles of what he called a “powerful new” wall on the border. “It’s the most powerful and comprehensive border wall structure anywhere in the world.”

Mr. Trump followed his border visit with a rambling 90-minute speech to a mostly maskless gathering of Students for Trump in Phoenix in which he vented about the removal of Confederate monuments, China’s handling of the coronavirus outbreak and one of his newer themes, voter fraud by mail.

He ratcheted up his usual predictions of fraud in the November election, made without any supporting evidence, by suggesting that mail-in ballots — which will be in more widespread use as Americans face limits on their movements because of the virus — were “a disaster for our country.”

He suggested at one point that mail carriers could be held up as they delivered ballots, which could then be counterfeited by enemies foreign and domestic.

“This will be, in my opinion, the most corrupt election in the history of our country,” the president said to the crowd, which booed at the mere mention of mailed ballots.

His speech also targeted demonstrators who have in recent weeks tried to tear down monuments of slaveholding Americans, many of them former leaders of the Confederacy.

“Lock ’em up. Lock ’em up,” he said of the demonstrators, repeating the signature phrase of his 2016 campaign when he threatened to jail his Democratic opponent, Hillary Clinton.

“You don’t burn buildings. You don’t punish dissenters. And you don’t erase the people with whom you disagree,” Mr. Trump fumed. “It’s called civilized people.”