Donald Trump on Saturday called Democrat Kamala Harris "mentally impaired" and said illegal immigrants are out to cut the throats of ordinary Americans in their own homes as he doubled down on the racially charged rhetoric fueling his unprecedented bid to regain the US presidency.

Trump was seeking to strike back at Harris after she visited the US-Mexico border on Friday and vowed to do more to control asylum claims and undocumented migrant crossings.  

The issue is one of the Democratic vice president’s weakest in polling, as she runs neck-and-neck against Trump in the November 5 election.

Trump dismissed Harris’s speech at the border, swearing and telling supporters in the small town of Prairie du Chien, Wisconsin, that President Joe Biden and Harris are responsible for an "invasion" of violent criminals.

Anti-immigrant sentiment has been at the core of Trump's appeal in economically depressed, majority-white parts of the country ever since his 2016 presidential victory, but the rhetoric is turning ever more extreme as election day nears.

The 78-year-old businessman — the first major presidential candidate in US history to be a convicted felon and subject of multiple ongoing court cases — said former California prosecutor Harris, 59, is "dumb."

"Joe Biden became mentally impaired. Kamala was born that way. She was born that way. And if you think about it, only a mentally disabled person could have allowed this to happen to our country," he said.

Harris, meanwhile, spent Saturday at a fundraiser in San Francisco, California, where she said Trump was using "the same tired playbook we've heard for years."

"This election is about two very different visions for our nation and we see that contrast on the campaign trail," she said.

Trump spoke in Wisconsin while flanked on stage by large mugshots of migrants accused of crimes, painting a hellish picture of an America under violent assault — despite a plunge this year in illegal border crossings and what the FBI says is a steep decline in crime overall, particularly murder.

"They will walk into your kitchen, they'll cut your throat," Trump said of illegal immigrants.

"Small towns in America are terrified of migrants coming in and, even when they haven't arrived, they’re terrified," he said. They will "rape, pillage, thieve, plunder and kill the people of the United States of America."

The illegal migrants, whom he called "animals," will take the jobs of minorities and of union workers, he added.

Apparently rankled by Harris’s bid Friday to deliver a tough message on the Mexican border, Trump cursed on the nationally streamed event and branded her "a liar."

He complained that Fox News, the conservative network which has provided him with a loyal platform throughout his White House career, had covered Harris's speech.

"They shouldn’t be allowed to put it on," he said.

Evidently, Trump was aware that his speech was unlike what Americans have been used to in presidential campaigns over the years.

He paused at one point to say: "Isn’t this a wonderful and inspiring speech? I’ve got people sitting in the front row — they’re going 'oh my God.'"

"This is a dark, this is a dark speech," he said.