US Vice President Kamala Harris assailed Donald Trump on Tuesday at her first campaign rally since replacing President Joe Biden as the Democratic presidential candidate, while a Reuters/Ipsos opinion poll showed her taking a marginal lead over Trump, the Republican nominee.

"In this campaign, I promise you I will proudly put my record against his any day of the week," she told a cheering crowd of several thousands at West Allis Central High School in a Milwaukee suburb in Wisconsin, a crucial battleground state in the November 5 election.

"Do we want to live in a country of freedom, compassion and rule of law, or a country of chaos, fear and hate?" she asked.

Harris led Trump 44 percent to 42 percent among registered voters in the national Reuters/Ipsos poll, conducted on Monday and Tuesday after Biden dropped out of the contest on Sunday and endorsed Harris as his successor.

Previous surveys taken before Biden's exit found Harris and Trump tied at 44 percent a week ago and Trump ahead of her by a percentage point at the beginning of the month.

In all three cases, the difference was within the poll's 3-point margin of error, but the results could signal some limited movement in Democrats' direction - and may suggest that Harris' elevation to the top of the ticket blunted whatever momentum Trump may have gained from last week's Republican National Convention, also in Milwaukee.

Harris swiftly consolidated her party's support after Biden, 81, abandoned his reelection campaign under pressure from members of his party who worried about his ability to beat Trump or to serve for another four-year term.

She wrapped up the nomination on Monday night by winning pledges from a majority of the delegates who at next month's party convention will determine the nominee, the campaign said.

Most Democratic lawmakers have lined up behind her candidacy, including the party's leaders in the Senate and House, Chuck Schumer and Hakeem Jeffries, who endorsed Harris on Tuesday at a joint press conference.

Harris' rise dramatically reshapes an election in which many voters were unhappy with their options.

Saddled with concerns that included his health and persistent high prices crimping Americans' household finances, Biden had been losing ground against Trump in opinion polls, particularly in the competitive states that are likely to decide the election, including Wisconsin and the Sun Belt states of Arizona and Nevada.

The Wisconsin event offered another opportunity for Harris, the first Black woman and Asian American to serve as vice president, to reset the Democrats' campaign.

Harris has been raking in campaign contributions. Her campaign said on Monday she had raised US$100 million since Sunday, topping the US$95 million that the Biden campaign had in the bank at the end of June.

Wisconsin is among a trio of Rust Belt states, along with Michigan and Pennsylvania, that are critical for Democrats' chances of defeating Trump.

"There are independents and young people who did not like their choices, and Harris has a chance to win them," said Paul Kendrick, executive director of the Democratic group Rust Belt Rising.

Milwaukee County Executive David Crowley, a Democrat, said Harris could also help bring back crucial Black voters.

"Many of them didn't come along because they were distracted by his (Biden's) age, distracted by his appearance," he said. (Reuters)