United States Vice President Kamala Harris is leading former President Donald Trump as she prepares for her headline moment at the Democratic National Convention, opinion polling shows.

On the eve of the convention in Chicago, Harris holds a lead of between three and six percentage points in the race for the White House, two opinion polls showed on Sunday.

Harris is ahead of Trump by 49-45 percent among registered voters and 51-45 percent among likely voters in a two-way race, according to an ABC News/Washington Post/Ipsos poll.

In a five-way race that includes Robert F Kennedy, Cornel West and Jill Stein, Harris leads Trump 47-44 percent among registered voters and 49-45 percent among likely voters.

In a CBS News poll, Harris is three points ahead nationally, leading Trump 51-48 percent.

In the seven battleground states considered pivotal to the outcome of the election – Pennsylvania, Michigan, Wisconsin, Arizona, Georgia, Nevada and North Carolina – Harris and Trump are tied 50-50.

The polls are the latest sign that Harris’s late entry into the race has energised Democrats and Democratic-leaning voters after months of poor polling for President Joe Biden, who dropped his re-election bid last month amid concerns about his fitness to serve a second term.

On Saturday, a New York Times/Siena College poll showed Harris in contention in the “Sun Belt” states of Arizona, Georgia, Nevada and North Carolina, which had been leaning heavily towards Trump when Biden was the Democratic nominee.

Before heading to Chicago on Sunday, Harris held several campaign events in Pennsylvania, whose 20 electoral votes flipped from Trump to Biden in 2020.

“I very much consider us the underdogs,” Harris said at a campaign stop in the township of Moon

“We have a lot of work to do to earn the vote of the American people. That’s why we’re on this bus tour today and we’re going to be travelling this country as we’ve been and talking with folks, listening to folks and hopefully earning their votes over the next 79 days.”

Speaking at a campaign in Pennsylvania a day earlier, Trump criticised Harris on inflation and her previous opposition to fracking, which is popular in the state, while also launching a series of personal attacks.

“People say, ‘Be nice’. Have you heard her laugh? That is the laugh of a crazy person… It’s the laugh of a lunatic,” Trump said.

While Harris and her running mate, Minnesota Governor Tim Walz, were formally nominated in a roll-call vote earlier this month, the four-day convention that kicks off on Monday will give Democrats an opportunity to display unity and drum up enthusiasm among voters.

Biden, former Presidents Bill Clinton and Barack Obama, former Secretary of State Hillary Clinton and former First Lady Michelle Obama are among the Democratic luminaries scheduled to address the convention.

Although Democrats have quickly coalesced around Harris, the Democratic nominee is expected to face demonstrations over the Biden administration’s support for the war in Gaza on the sidelines of the convention.

Dozens of Democratic delegates calling themselves “Delegates Against Genocide” have said that they will use the convention to press for an embargo on US arms sales to Israel.

The Democrats’ draft platform released last month calls for an immediate ceasefire in the war, without mentioning the Palestinian death toll or calling for a halt to US arms shipments to Israel.