The day after Vice President Kamala Harris visited the southern border and pledged to crack down on asylum and beef up security, former President Donald J. Trump unleashed a string of sharply personal attacks on her at a rally on Saturday, expressing contempt for her intelligence and calling her “mentally disabled.”

In a dark, often rambling speech lasting longer than an hour, Mr. Trump — whose advisers have urged him to focus on policy issues rather than on personal jabs — notably escalated his attacks against Ms. Harris. Mr. Trump, who has often questioned President Biden’s mental abilities, told supporters at a rally in Prairie du Chien, Wis., that “Joe Biden became mentally impaired; Kamala was born that way.”

Mr. Trump then tied Ms. Harris to the Biden administration’s border policies, adding, “And if you think about it, only a mentally disabled person could have allowed this to happen to our country.” Later, he criticized her remarks at the border on Friday as “bullshit.”

It was a startling series of broadsides in the midst of a presidential campaign, even for a candidate who seems to delight in offensive remarks. Mr. Trump’s speech in Prairie du Chien, a town of about 5,000 people along the Mississippi River, was meant to serve as a response to Ms. Harris’s border visit, in Douglas, Ariz., where she promised to crack down on asylum and called for tougher punishments against those who cross the border illegally. Those positions, an attempt to address a political vulnerability, made up the core of one of the toughest speeches on immigration and border policy that a Democrat has made in a generation.

But Mr. Trump, who stood surrounded by posters of undocumented immigrants accused of violent crimes, attacked Ms. Harris for being a political opportunist. And he claimed that she bore responsibility for migrants who have come into the country illegally and committed crimes.

“She is a disaster,” Mr. Trump said. “And she is not ever going to do anything for the border, and she didn’t even want to get tough now, except her poll numbers were tanking.”

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