US Democratic presidential candidate Kamala Harris will join celebrity billionaire Oprah Winfrey for an online event with grassroots groups on Thursday to rally support in battleground states.

The "Unite for America" event, co-hosted by Winfrey and activist group Win with Black Women, aims to register people to vote and bolster Harris in states such as Pennsylvania, Georgia and Michigan that are set to decide the Nov 5 election.

Harris and Winfrey will livestream the 8pm EDT (0000 GMT) event on social media including Instagram and TikTok from Michigan, where Harris has been campaigning in the Detroit area, local press reported.

Dozens of grassroots groups including Latinas for Harris, Win With Black Men and Republicans for Harris are participating.

Win with Black Women campaigns for Black women to win elected offices. Harris is Black and has South Asian heritage.

The groups, which rallied around Harris since she entered the race in July, have helped raise more than US$20 million for her campaign, a Unite for America press release said.

"What is essential to me is getting people motivated to vote, and ensuring everyone makes a plan not only for themselves, but for their families and friends," Winfrey, a businesswoman, philanthropist and former TV host, said in a statement.

In a recent Reuters poll, Harris led her Republican opponent Donald Trump 47 per cent to 42 per cent.

Harris was ahead in the battleground states of Michigan, Pennsylvania, Wisconsin, Nevada and North Carolina, while Trump had greater support in Georgia, according to polling averages from aggregator FiveThirtyEight. They are tied in Arizona.

Trump on Thursday will participate in a Washington event on fighting antisemitism and address the Israeli-American Council Summit.

Hours after President Joe Biden dropped out and endorsed Harris to be the Democratic candidate, 90,000 Black women joined a call held by Win with Black Women that raised US$1.5 million in three hours, the press release said.

In August, Winfrey spoke at the Democratic National Convention in Chicago to press the case for Harris.

"I'm calling on all you independents and all you undecided," Winfrey said then, describing herself as an independent voter. "Decency and respect are on the ballot in 2024."

  - Reuters