President Donald Trump savaged his Democratic opponent Joe Biden on Tuesday at the start of a tour of swing states, but seemed to concede that his reelection campaign is running out of donors, requiring him to dip into his own pocket.

"If I have to, I will," Trump told reporters when asked about spending his own cash. "Whatever it takes. We have to win."

Trump was on form at a rally with supporters in Winston-Salem, North Carolina, which he visited immediately after a speech in Florida -- two of the battleground states he almost certainly has to win to beat Biden on November 3.

After attacking Biden, he reserved his especially acid remarks for Kamala Harris. "You know what, people don't like her. Nobody likes her," said Trump, who also pointedly mispronounced Harris' first name, a gesture common on the right and widely interpreted as a way of demonstrating disrespect.

"She could never be the first woman president. That would be an insult to our country," he said.

His remarks came as a Reuters/Ipsos opinion poll showed Biden leads Trump by 12 percentage points nationally among likely US voters.

The Sept. 3-8 poll, released yesterday, found 52% of likely voters planned to support Biden, while 40% would back Trump. Three percent said they would vote for another candidate, and just 5% said they remained undecided with less than two months to go until the Nov 3 presidential election.