US president Joe Biden and his leading Republican challenger Donald Trump remain locked in a tight election rematch, after a special prosecutor’s report commenting on the Democratic incumbent’s mental acuity, a Reuters/Ipsos poll found.

The four-day poll, which closed on Monday, showed former president Trump with the support of 37 per cent of respondents, compared with 34 per cent support for Mr Biden, at the edge of the survey’s 2.9 percentage point margin of error.

Some 10 per cent said they would vote for other candidates; 12 per cent said they would not vote; and 8 per cent refused to answer the nationwide poll conducted online with responses from 1,237 US adults.

The poll was conducted in the days after special counsel Robert Hur released a report that declined to charge Mr Biden for taking classified documents when he left the vice presidency in 2017, but also described him as a “well-meaning, elderly man with a poor memory”.

The White House has sharply disputed that characterisation.

The finding showed a closer race than a poll last month that found Mr Trump holding a 6 percentage point lead.

It also showed that one potential major political liability facing Mr Trump is the ongoing four criminal prosecutions he faces. One in four self-identified Republicans and about half of independents responding said they would not vote for Mr Trump if he was convicted of a felony crime by a jury.

A nationwide survey does not capture the subtleties of the electoral college contest that will be decided this fall in just a handful of competitive states. The Electoral College results are the final determinant of the presidential election outcome.