US Vice President Kamala Harris has attacked Donald Trump at her first campaign rally since replacing President Joe Biden as the Democratic presidential candidate.
The event came as a Reuters/Ipsos opinion poll showed her taking a marginal lead over Mr 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 thousand people at a high school in Milwaukee, Wisconsin, a crucial battleground state in the 5 November 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.
Ms Harris leads Mr Trump 44% to 42% among registered voters in the national Reuters/Ipsos poll, conducted today and yesterday, after President Biden dropped out of the contest on Sunday and endorsed his vice president as his successor.
Previous polls taken before Mr Biden's exit found that Ms Harris and Mr Trump tied at 44% a week ago and Mr Trump ahead of her by 1% at the beginning of the month.
In all three cases, the difference was within the poll's three-point margin of error, but the results could signal some limited movement in Democrats' direction.
They may also suggest that Ms Harris' elevation to the top of the ticket hit whatever momentum Mr Trump may have gained from last week's Republican National Convention, also in Milwaukee.
Ms Harris swiftly consolidated her party's support after President Biden, 81, abandoned his reelection campaign under pressure from members of his party who worried about his ability to beat Mr Trump or to serve another four-year term.
She wrapped up the nomination last 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 politicians have lined up behind her candidacy, including the party's leaders in the US Senate and House of Representatives, Chuck Schumer and Hakeem Jeffries, who endorsed Ms Harris today.
An unofficial survey of delegates by the Associated Press showed Ms Harris with more than 2,500 delegates, well over the 1,976 needed for the nomination.
Delegates could still change their minds, but no one else received any votes in the survey while 54 delegates said they were undecided.
Ms Harris' rise dramatically reshapes an election in which many voters were unhappy with their options.
Saddled with concerns that include his health and persistent high prices crippling Americans' household finances, President Biden had been losing ground against Mr Trump in opinion polls, particularly in the competitive states that are likely to decide the election, including Wisconsin, Arizona and Nevada.
The Wisconsin event offered another opportunity for Ms Harris, the first black woman and Asian American to serve as vice president, to reset the Democrats' campaign.
Ms Harris has been raking in campaign contributions. Her campaign said that she had raised $100 million (€92m) since Sunday, topping the $95m (€88m) that the Biden campaign had in the bank at the end of last month.
While a wave of senior Democrats have lined up behind Ms Harris, the racial justice group Black Lives Matter challenged the party's swift move.
It called for a national virtual snap primary election ahead of the Democratic National Convention in Chicago from 19 to 22 August, where the party will formally nominate its candidate.
"We call for the Rules Committee to create a process that allows for public participation in the nomination process, not just a nomination by party delegates," Black Lives Matter said in a statement.
President Biden said on social media that he would deliver a speech tomorrow night from the Oval Office explaining his decision to end his campaign.
He has returned to Washington after spending several days in isolation at home with Covid-19.
The president tested negative and no longer has symptoms, the White House doctor said.
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Mr Biden's exit followed Mr Trump's narrow survival of a 13 July assassination attempt that raised questions about security failures in the US Secret Service.
Agency Director Kimberly Cheatle resigned after numerous politicians called for her to step down.
Mr Trump and his allies have tried to tether Ms Harris to some of President Biden's more unpopular policies, including his administration's handling of the surge of migrants at the southern border with Mexico.
"Kamala Harris' dismal record is one of complete failure and utter incompetence. Her policies are Biden's policies, and vice versa," Trump spokesperson Steven Cheung said.
Wisconsin is among a trio of states, along with Michigan and Pennsylvania, that are critical for Democrats' chances of defeating Mr Trump.
Milwaukee County Executive David Crowley, a Democrat, said that Ms 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.
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Democratic National Committee Chair Jaime Harrison, in an interview on NBC's "Today" programme, said the party had to move quickly to get the ticket on ballots in all 50 states, and that the vice presidential pick needed to be made by 7 August.
"This process is going to be fair, transparent, open but it's going to be fast," Mr Harrison said.
Potential running mates include Kentucky Governor Andy Beshear, US Secretary of Transportation Pete Buttigieg, North Carolina Governor Roy Cooper, Arizona Senator Mark Kelly, Illinois Governor JB Pritzker, Pennsylvania Governor Josh Shapiro and Michigan Governor Gretchen Whitmer.
Ms Harris' selection of Milwaukee for today's rally is no accident. Last week, the city played host to the Republican National Convention, where Mr Trump was nominated as the party's official candidate for president.
With the US role as a world power under scrutiny amid the political chaos, Mr Biden and Ms Harris will also have to deal with a major foreign policy headache this week.
They are due to hold separate meetings with Israeli Prime Minister Benjamin Netanyahu as he visits Washington, with President Biden saying he will spend his remaining time in office working to end the war in Gaza.
Ms Harris said she would not attend Mr Netanyahu's address to the US Congress tomorrow due to a pre-planned rally, but an aide said this "should not be interpreted as a change in her position with regard to Israel".
Mr Trump said that he will meet Mr Netanyahu at his Mar-a-Lago estate in Palm Beach, Florida on Friday.
