Donald Trump came under fire for violent remarks targeting a high-profile Republican supporter of Kamala Harris Friday as the candidates held rallies in critical Rust Belt battleground states four days before the climax of a volatile US presidential campaign.

More than 68 million Americans have already cast their ballots ahead of Election Day on Tuesday. Opinion polls show Trump and Harris running dead even, with victory depending on who prevails across the seven swing states, including Wisconsin and Michigan.

Both held a full day of campaign events, concluding with rallies in Wisconsin's largest city Milwaukee.

"Everything we have been fighting for these past four years comes down to the next four days," Trump told a boisterous crowd in Warren, Michigan.

The former president then was to speak in the same venue where he earned the Republican Party nomination over the summer and gave a triumphant acceptance speech just days after surviving an assassination attempt in Pennsylvania.

Harris -- who only entered the race in July after President Joe Biden dropped out amid fears over his declining mental acuity -- was to be joined by star rapper Cardi B in the latest of a series of high-energy rallies.

Earlier, Harris lambasted Trump for using "violent rhetoric" when discussing one of his chief Republican critics.

Trump has "suggested rifles should be trained on former representative Liz Cheney," Harris told reporters in Madison, Wisconsin.

"This must be disqualifying. Anyone... who uses that kind of violent rhetoric is clearly disqualified and unqualified to be president."

'Vote, vote, vote'

As the contest draws to a close, Trump has doubled down on his most provocative talking points, seeking to fire up a base he needs to turn out in massive numbers.

"Get everyone you know. Vote, vote, vote," the 78-year-old billionaire pleaded in Michigan, which with Wisconsin is part of the Democrats' "blue wall" across the Midwest.

But the region could go either way -- and with it, the presidency.

The other path to victory could run through southern and western Sun Belt swing states, where Trump and Harris both campaigned Thursday.

At an Arizona event with former Fox News host Tucker Carlson, Trump called Harris, 60, a "sleaze bag."

He also claimed, without evidence, that polls are being rigged in the biggest swing state Pennsylvania -- reinforcing expectations that, as in 2020, he will refuse to accept the results if he loses.

But it was his comments about Cheney, once a senior Republican lawmaker who is backing Harris, that stirred the most controversy.

Citing her hawkish foreign policy views, Trump conjured the image of Cheney -- daughter of former Republican vice president Dick Cheney -- being shot.

"She's a radical war hawk. Let's put her with a rifle standing there with nine barrels shooting at her, OK? Let's see how she feels about it, you know, when the guns are trained on her face," Trump said.

Cheney responded, saying, "This is how dictators destroy free nations. They threaten those who speak against them with death." 

Harris rushed to her defense, saying Trump is increasingly unhinged and "permanently out for revenge."

But Trump did not back down, saying Cheney "doesn't have the guts to fight" America's enemies.

Adding to tensions, social media is awash with disinformation that authorities say has been stirred by Russian operatives and amplified by prominent right-wing voices -- including Trump ally Elon Musk, the world's richest man and owner of the X platform.

Much of Musk's effort has involved pushing lies about non-citizen immigrants voting.

'Junk science'

Trump notably made a stop Friday in Dearborn, Michigan -- home to the largest Arab-American community in the country, where outrage over Israel's war in Gaza has alienated many Muslims from the Democratic Party.

After meeting supporters at a halal restaurant, Trump confirmed to reporters that vaccine skeptic Robert F. Kennedy Jr would play a "big role" in health care if he were elected. Kennedy later joined Trump on stage in Warren.

Harris described Kennedy as a promoter of "junk science and crazy conspiracy theories."

Harris has repeatedly warned about the dangers of an authoritarian Trump administration.

For a few minutes in Michigan, Trump's oratory suggested a traditional rousing speech in the final days of a campaign.

"I'm asking you to dream big again," he told the crowd. 

But he quickly reverted to type with a sinister take on immigration and falsely branding the Biden-Harris economy a "total disaster."

Economists say the US economy is actually in robust shape, shrugging off the last Covid pandemic cobwebs, with low unemployment and strong growth.