Democrat Joe Biden leads President Donald Trump by 12 percentage points nationally among likely US voters, according to a Reuters/Ipsos opinion poll.

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

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

Fact check: President Trump makes 10 false and misleading claims about Joe Biden during North Carolina rally https://t.co/xGi9I57LRD pic.twitter.com/reHDTq9WCc

— CNN Politics (@CNNPolitics) September 9, 2020

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 Mr Biden currently had the advantage in securing the national popular vote.

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

Mr 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 three million more votes than Mr 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, Mr Biden leads Mr Trump by 8 percentage points, versus his seven-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 Mr Biden would be better at handling the US coronavirus response, while 38 per cent said Mr Trump would be better.

But Mr 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 Mr Trump, while 40 per cent said Mr 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 Mr Biden would be better for the national economy and expanding the workforce, while 45 per cent said they thought Mr 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 four percentage points.

-AAP