In the battleground state of Wisconsin on Saturday, Trump called Democratic Vice President Harris, - who on Friday visited the US-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 US 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 November 5 election. Immigration and the southern border are one of the top issues for voters, according to opinion polls.
The former president blamed Harris and Democratic President Joe Biden for allowing undocumented immigrants into the United States, 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 US illegally was detained in September for allegedly sexually assaulting a woman and attacking her daughter.
Some seven million migrants have been arrested crossing the US-Mexico border illegally during Biden's administration, according to government data, a record high number that has fuelled 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 US.
Trump also repeated his false claim that his 2020 election defeat to Biden was fraudulent. If re-elected, 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.
Kamala Harris says Donald Trump fans "the flames of fear and division" over the impact of immigrants (AP PHOTO)
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 US over the southern border.
Several parents who had lost children to attacks by immigrants in the US illegally, or to fentanyl, spoke in support of him before Trump's remarks on Saturday.
