The poll also shows the number of persuadable voters has shrunk compared with four years ago.

The September 3-8 poll, released on Wednesday, found 52 per cent of likely voters planned to support Biden, while 40 per cent would back Trump.

Three per cent said they would vote for another candidate, and just 5 per cent said they remained undecided with less than two months to go until the November 3 presidential election.

The survey showed the number of voters who had not yet backed a major-party candidate to be less than half of what it was in 2016, and that Biden currently had the advantage in securing the national popular vote.

Even if the remaining undecided voters threw their support behind Trump, the poll showed, he would still lose the popular vote to Biden.

Trump can still win re-election, however, without winning the national popular vote. US presidential elections are not decided by the national vote but rather who wins the Electoral College, a contest based on a tally of wins from state-by-state contests.

Four years ago, Democrat Hillary Clinton got almost 3 million more votes than Trump, only to see her Republican rival narrowly win the Electoral College and the presidency.

This was the first time the Reuters/Ipsos poll measured support for the 2020 candidates among likely voters.

When measured by registered voters who include those less likely to vote, Biden leads Trump by 8 percentage points, versus his 7-point lead in a similar poll last week.

The poll showed likely voters being primarily motivated by the coronavirus pandemic, which has killed more than 186,000 Americans and put millions out of work, and restoring trust in government.

When asked what was driving their pick for president, 28 per cent said it was the candidate's perceived ability to handle the coronavirus, and 23 per cent said it was the ability to restore trust in government.

An additional 19 per cent said it was the candidate's ability to boost the economy, and 14 per cent said they were looking for a candidate who is "tough on crime."

Fifty-one per cent of likely voters said Biden would be better at handling the US coronavirus response, while 38 per cent said Trump would be better.

But Trump has the edge when it comes to their perception of who would be "tough on crime and civil unrest," with 45 per cent choosing Trump, while 40 per cent said Biden would be better.

On the economy, neither candidate has the upper hand among likely voters: 45 per cent of likely voters said they thought Biden would be better for the national economy and expanding the workforce, while 45 per cent said they thought Trump would be best.

The Reuters/Ipsos poll was conducted online and gathered responses from 823 likely voters, including 390 who identified as Democrats and 351 who identified as Republicans. The poll has a credibility interval, a measure of precision, of about 4 percentage points.

Australian Associated Press