President Donald Trump will announce a new, $13 bn round of aid to US farmers at a campaign rally in Wisconsin on Thursday night, the Reuters news agency reported citing an unnamed person familiar with the president's plan.

It is a battleground state in Trump's presidential contest with Democrat Joe Biden.

Trump's margin of victory in Wisconsin in 2016 over Democrat Hillary Clinton was by less than 1 percent of the vote and marked the first time the state went for a Republican presidential candidate since 1984.

This weekend, $5.4m of that is being spent to run new television ads in the battleground state of Florida that slam Donald Trump's response to the pandemic, according to McClatchy DC.

A former top aide to Vice President Mike Pence, Olivia Troye, has accused US President Donald Trump of not taking the novel coronavirus seriously earlier this year.

It was terrifying," said Troye, who left the White House in July, in an online video advertisement produced by Republican Voters Against Trump, about the administration's response to the global pandemic.

Democratic presidential candidate Joe Biden said US President Donald Trump is trying to de-legitimise the upcoming November election and is encouraging foreign interference.

Terry Branstad, a former Republican governor of Iowa, told a Hong Kong television outlet that he will work to help President Donald Trump, Senator Joni Ernst and others win in Iowa.

"My son is very involved in [Trump's] campaign, and I will be a volunteer to help him, to help Joni Ernst and other friends for the election in Iowa," said Branstad, stressing that he will not hold an official role "but ...

"I've tried to provide an honest accounting of my presidential campaign and my time in office: the key events and people who shaped it, my take on what I got right and the mistakes I made, and the political, economic, and cultural forces that my team and I had to confront then - and that as a nation we are grappling with still," Obama said in a statement.

Democrat Joe Biden's campaign released two new ads Thursday, one featuring a Pennsylvania Biden backer who voted for Trump in 2016 and who is not pleased with the president's handling of the coronavirus pandemic.

"I would be very scared if Joe Biden became president," begins a new advertisement from Donald Trump's presidential campaign focused on the economy.

A diverse group of Trump supporters participating in what the campaign calls an "economic roundtable" took turns trashing what a Biden presidency would do Americans' economic security.

US President Donald Trump posted a strongly worded tweet claiming, without evidence, that the election results may be illegitimate due to "unsolicited ballots" being sent to voters.

Following a recent rash of polling, the Cook Political Report, which handicaps US elections, said Trump's problems in the key battleground state of Arizona are mounting.

Trump won Arizona in 2016 by 3.5 percent over Hillary Clinton.

Suburban Phoenix voters have "soured" on Trump, Arizona Republican voters "are not as committed to voting for him", and "Trump is also struggling with Latino support," writes Amy Walter of the Cook Political Report.

Walter cites a new poll showing Biden leading Trump 45 percent to 40 percent in Arizona and writes, "The new data in this poll, combined with other recent polling in the state, all find Arizona slipping away from Trump.".

A day after spending the day in their respective home cities, President Donald Trump and his Democratic rival Joe Biden will hit the road on Thursday for campaign events in key battleground states

A series of polls released this week show Biden maintaining a lead in Wisconsin, a state Trump won by a slim 0.77 percent margin over Hillary Clinton in 2016