Kamala Harris and Donald Trump enter the final weekend of the most tense US presidential campaign of modern times with a flurry of swing-state rallies that will test their stamina -- and ability to persuade the country's last undecided voters.

Harris, bidding to become the country's first woman president, will use rallies in Georgia, North Carolina and Michigan to drive home her message that Trump is a threat to US democracy.

Trump -- seeking a sensational return to the White House after losing in 2020 and then becoming the first presidential nominee to have been convicted of crimes -- promises a radical right-wing makeover of the government and aggressive trade wars to promote his policy of "America first."

The 78-year-old, who rallied in Milwaukee, Wisconsin late Friday just miles from Harris's event there, will all but cross paths with her again as Trump makes whistle-stops in North Carolina, Virginia, Pennsylvania and Georgia.

Their frenetic schedule will run right into Monday, culminating with late-night rallies -- in Grand Rapids, Michigan, for Trump and Philadelphia, Pennsylvania, for Harris.

Election Day is Tuesday but Americans have been voting early for weeks, with more than 70 million ballots already cast -- including a record four million in Georgia, where Democrats seek to pull out all the stops to keep the state in their column.

Opinion polls continue to show a tied race, particularly in the seven battleground states likely to determine the result in the US electoral college system, leaving the Republican businessman and his 60-year-old Democratic rival fighting hard to peel off even slivers of support from one another's camps.

Harris, currently President Joe Biden's vice president, is doing that by appealing to centrist voters and propelling her base to the polls with a robust ground game and get-out-the-vote effort.

And by painting Trump as a toxic authoritarian, she is also encouraging voters to "finally turn the page" on the former president.

"He is someone who is increasingly unstable, obsessed with revenge, consumed with grievance -- and the man is out for unchecked power," she told supporters in Little Chute, Wisconsin.

- 'Thrill of a lifetime' -

Trump, meanwhile, has doubled down on his already extreme rhetoric in hopes of firing up his loyal base to turn out in massive numbers.

"Kamala's closing message to America is that she hates you," Trump fumed on Friday night in Warren, Michigan, where he trashed the economy under Biden and Harris as a disaster -- which economists say it clearly is not -- and warned that "a 1929-style economic depression" would ensue if Harris were elected.

Citing her hawkish foreign policy views, Trump earlier had conjured the image of former Republican representative turned Harris supporter Liz 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.

Despite the rhetoric, Trump waxed nostalgic on Friday about how his experience campaigning over the past nine years has been "the thrill of a lifetime."

"And now we want to take that thrill and turn it into 'let's do business,' right?" 

Harris, the nation's first Black and first Asian-American vice president, meanwhile has sought to harness celebrity star power like Beyonce and Bruce Springsteen in the campaign's waning days.

Jennifer Lopez, a pop icon of Puerto Rican heritage, joined Harris onstage Thursday, amid a firestorm triggered by a Trump rally warm-up speaker branding the US territory a "floating island of garbage." 

Grammy-winning rapper Cardi B appeared with the candidate Friday night, asking the crowd in Milwaukee, "Are we ready to make history?"

With the election just days away -- and Trump refusing to say whether he would accept its results if he loses -- businesses in the capital Washington have begun boarding up shop fronts as city authorities warn of a "fluid, unpredictable security environment" in the days after the polls close.

Trump is already alleging fraud and cheating in swing states such as Pennsylvania, laying the groundwork for what many fear will be more unrest, following the violence that erupted at the US Capitol in the wake of the 2020 vote.