US Republican presidential candidate Donald Trump has attacked his new Democratic rival Kamala Harris as the “ultra-liberal driving force” who should be blamed for the Biden administration’s policies on immigration.

Trump addressed a North Carolina rally on Wednesday, three days after President Joe Biden abruptly dropped his re-election bid and endorsed Harris, his vice president.

She has since pulled in broad support across the Democratic Party and revitalised its election campaign.

Trump tried to quash some of that momentum in an aggressive speech at the rally, his first since Harris’ emergence changed the race.

“She is a radical left lunatic who will destroy our country if she ever gets elected,” he said.

Trump routinely uses insults in attacking his opponents and made clear he planned to ignore advice that he take a softer line.

“I’m not gonna be nice!” he told his cheering supporters in Charlotte.

Harris, the first black woman and Asian-American to serve as vice president, would become the first woman elected president if she prevails on November 5.

Trump’s rally began two hours before Biden, 81, addressed the nation from the Oval Office to explain why he dropped out of the race under mounting pressure from fellow Democrats.

As his campaign signalled earlier this week, Trump focused his attack on Harris around the immigration issue.

“As border tsar, Kamala threw open our borders that allowed 20 million illegal aliens to stampede into our country from all over the world,” Trump said.

“I will terminate every single open border policy of the Biden-Harris administration, and we will seal the border and we will stop Kamala Harris’s invasion without delay.”

Biden put Harris in charge of working with countries in Central America to help stem the tide of migration. But she was not given responsibility for border security, nor was she named “border tsar.”

Trump also touched on abortion, an issue that Democrats have long seen as a winning issue for them.

“When you compare my position on abortion to that of Kamala Harris, my position is eight points higher in the polls. And that’s because she is so radical,” he said.

Harris has been the point person for the Biden administration on the issue of abortion, which is expected to become even more central to the campaign with her at the top of the ticket.

Trump, coming off a triumphant week in which his party unified around his presidential bid after a failed assassination attempt two weekends ago, has had to watch as Biden’s sudden departure from the race shifted the narrative and sparked a surge of attention toward Harris at his expense.

Harris and Trump are closely competitive, public opinion polls showed this week.