Donald Trump on Saturday deployed some of his harshest rhetoric against immigrants who have crossed the border illegally and committed crimes, especially against young women, while he also escalated his personal insults against Kamala Harris.

In the battleground state of Wisconsin, Trump called Democratic Vice President Harris, who on Friday visited the U.S.-Mexico border for the first time in her 2024 presidential campaign, "mentally impaired" and "mentally disabled".

The Republican presidential candidate was flanked by posters of immigrants in the U.S. illegally who have been arrested for murder and other violent crimes, and banners saying "End Migrant Crime" and "Deport Illegals Now."

His speech was unusually devoted almost entirely to undocumented immigrants. He called those who had committed violent crime "monsters," "stone-cold killers" and "vile animals".

Trump is locked in a close race with Harris ahead of the Nov. 5 election. Immigration and the southern border are one of the top issues for voters, according to opinion polls.

Sarafina Chitika, a Harris spokesperson, said after Trump's speech: "He's got nothing 'inspiring' to offer the American people, just darkness."

The former president blamed Harris and Democratic President Joe Biden for allowing undocumented immigrants into the U.S., accusing some migrants of wanting to "rape, pillage, thieve, plunder and kill the people of the United States of America."

At one point Trump admitted: "This is a dark speech".

Trump's speech was in the small Wisconsin city of Prairie du Chien, where a Venezuelan in the U.S. illegally was detained in September for allegedly sexually assaulting a woman and attacking her daughter.

Some 7 million migrants have been arrested crossing the U.S.-Mexico border illegally during Biden's administration, according to government data, a record-high number that has fueled criticism of Harris and Biden from Trump and fellow Republicans.

In her visit to the border on Friday, Harris outlined her plans to fix "our broken immigration system" while accusing Trump of "fanning the flames of fear and division" over the impact of immigrants on American life.

Harris also called for tighter asylum restrictions and vowed to make a "top priority" of stopping fentanyl from entering the U.S.

Trump also repeated his false claim that his 2020 election defeat to Biden was fraudulent. If reelected, and "if allowed", Trump said he would prosecute people he blames for his loss then.

Studies generally find there is no evidence immigrants commit crimes at a higher rate than native-born Americans and critics say Trump's rhetoric reinforces racist tropes.

Trump typically focuses on young women allegedly killed by Hispanic assailants to drive home that message, eschewing cases that involve male victims.

Trump's opponents accuse him of cynically exploiting grieving families to fuel his narrative that foreign-born, often Hispanic, arrivals are part of an invading army.

But some of the families of the victims have welcomed Trump's focus on the issue of violent crime and the death toll of teenagers caused by the opioid drug fentanyl, much of which crosses into the U.S. over the southern border.

Several parents who had lost children to attacks by immigrants in the U.S. illegally, or to fentanyl, spoke in support of him before Trump's remarks on Saturday.