Milwaukee, Wisconsin - President Joe Biden called Donald Trump a "loser" Wednesday, in his first battleground campaign stop since the two rivals secured their parties' nominations for the 2024 election.

Speaking to supporters in Milwaukee, Wisconsin, the Democrat also blasted his hard-right Republican nemesis for describing immigrants as "vermin."

The return to the campaign trail came a day after Biden and Trump both won enough delegates to clinch their parties' nominations for a rematch in November.

"A lot of you helped me in 2020 and we made sure he was a loser. And we're going to make sure that happens again, right?" Biden told local supporters and volunteers in Milwaukee – the same city where Trump and his party will hold the Republican National Convention in July.

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Biden has taken to repeatedly calling Trump a loser, knowing it rankles the former president, who still refuses to acknowledge he lost four years ago.

Wisconsin and Michigan, which Biden will visit on Thursday, were among the crucial states he flipped from Trump in their 2020 showdown and needs to win again to secure a second term.

The 81-year-old took on his rival over immigration as Trump resorts to increasingly violent anti-migrant rhetoric.

"We are a country of immigrants. They're not vermin," Biden said, referring to comments Trump made last year. Trump has also referred to immigrants "poisoning the blood of our country."

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Biden also addressed the issue of marijuana reform, which he called a priority in his State of the Union speech last week.

Earlier in Milwaukee, Biden unveiled over $3 billion in infrastructure investment to help "left behind" areas as he seeks to tout what he calls an "American comeback" for the economy.

The attacks underscored how bitter the next eight months of US politics promise to be, with the country's oldest ever pair of election candidates making no secret of the personal bitterness between them.

Both men lashed out at each other after their nomination wins on Tuesday.

Trump called Biden the "Worst, Most Incompetent, Corrupt and Destructive President in the History of the United States."

Biden assailed Trump's "campaign of resentment, revenge, and retribution" and branded him a threat to democracy.

If reelected, Trump has pledged to be a "dictator" for his first day in power, when he says he will shut the border, order oil drilling and release supporters jailed for the January 6, 2021 US Capitol attack.

He has also promised the biggest ever mass deportation of millions of immigrants.