NEW YORK: US Vice President Kamala Harris opened up a marginal two-percentage-point lead over Republican Donald Trump after President Joe Biden ended his re-election campaign and passed the torch to her, a Reuters/Ipsos poll found.

That compares with a marginal two-point deficit Biden faced against Trump in last week's poll before his Sunday exit from the race.

The new poll, conducted on Monday and Tuesday, followed both the Republican National Convention where Trump on Thursday formally accepted the nomination and Biden's announcement on Sunday he was leaving the race and endorsing Harris.

Harris, whose campaign says she has secured the Democratic nomination, led Trump 44 per cent to 42 per cent in the national poll, a difference within the three-percentage-point margin of error.

Harris and Trump were tied at 44 per cent in a July 15-16 poll, and Trump led by one percentage point in a July 1-2 poll, both within the same margin of error.

While nationwide surveys give important signals of American support for political candidates, a handful of competitive states typically tilt the balance in the US Electoral College, which ultimately decides who wins a presidential election.

A pollster with Trump's campaign played down any polling showing an increase in Harris' support, arguing that she was likely to see a temporary rise in popularity because of widespread media coverage of her new candidacy.

"That bump is likely to start showing itself over the next few days and will last for a while," pollster Tony Fabrizio said in a memo circulated to reporters by Trump's campaign.

Candidates often expect a bump after formally accepting their party's nomination at stage-managed, televised conventions such as the one that Trump had last week. But the poll showed no sign of that.

Bump or not, the most recent Reuters/Ipsos poll underscored the rationale for Biden dropping out of the race and for Harris replacing him on the ticket.

Some 56 per cent of registered voters agreed with a statement that Harris, 59, was "mentally sharp and able to deal with challenges," compared to 49 per cent who said the same of Trump, 78.

Only 22 per cent of voters assessed Biden that way. Biden, 81, ended his reelection effort after a debate with Trump in which he often stammered and failed to aggressively challenge attacks by Trump that included falsehoods.

Some 80 per cent of Democratic voters said they viewed Biden favorably, compared to 91 per cent, who said the same of Harris. Three quarters of Democratic voters said they agreed with a statement that the party and voters should get behind Harris now, with only a quarter saying multiple candidates should compete for the party's nomination.

When voters in the survey were shown a hypothetical ballot that included independent presidential candidate Robert F. Kennedy Jr., Harris led Trump 42 per cent to 38 per cent, an advantage outside the margin of error. Kennedy, favored by 8 per cent of voters in the poll, has yet to qualify for the ballot in many states ahead of the Nov. 5 election.

Harris campaigned in the critical battleground state of Wisconsin on Tuesday. She won support from major party figures and attention has since turned to who she will pick as her running mate.

Many respondents in the Reuters/Ipsos poll said they knew nothing about the Democrats seen as potential picks to join Harris' ticket.

About one in four registered voters said they had never heard of US Transportation Secretary Pete Buttigieg, a former Democratic presidential candidate who had the highest favorability rating - 37 per cent - of the potential Harris running mates in the poll.

One in three had not heard of California Governor Gavin Newsom, with about the same share saying they looked on him favorably.

Half of registered voters in the poll had never heard of Arizona Senator Mark Kelly and two-thirds knew nothing of Kentucky Governor Andy Beshear.

The poll, which was conducted online, surveyed 1,241 people.

 - Reuters