Donald Trump has overtaken Hillary Clinton for the first time since mid-July, according to a weekly Rasmussen poll published on Thursday.

Trump is polling at 40 per cent, a point ahead of Clinton at 39.

It's a striking reversal of fortunes for the two candidates compared to last week, when Clinton led Trump 42 to 38.

The result comes as Labor Day approaches. Following the holiday presidential campaigns ratchet up several notches, so polls then are seen as giving whoever is ahead at that point a crucial psychological advantage.

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Trump has moved ahead of Hillary in the latest poll from Rasmussen, at 40 per cent to Clinton's 39

Hillary's comfortable lead has been eroded in the past week, just days before the crucial Labor Day watershed

Rasmussen said that a one-point lead is statistically insignificant due to the poll's 3 percentage point margin of error, but said the results highlighted the closeness of the race.

But two separate polls also showed that Clinton's lead has shrunk.

A poll by Fox News had Clinton 6 points ahead at 48 per cent against Trump's 42 - but that gap has shrunk from the 10-point lead she had earlier this month.

And on Wednesday a Reuters/Ipsos poll also showed a dramatic narrowing between the candidates. Clinton now polls at 40 per cent to Trump's 39 per cent - a major reduction of her 42-35 lead the previous week.

Clinton's momentum has slowed since her 44 per cent spike in support in early August following the Democratic National Convention.

The Democrat has been unable to shake off the ongoing scandal over her use of a personal email server to conduct state business, as well as questions over whether she inappropriately used her position as secretary of state to help fill the coffers of the Clinton Foundation.

The Rasmussen poll put Trump slightly ahead

According to the Rasmussen poll both candidates have lost support this week from their party voters. Trump's following among Republicans has fallen to 71 per cent, from 76 last week; Clinton is at 73 per cent, down from 79 previously.

The Rasmussen survey of 1,000 likely voters was conducted on August 29-30. It is done using automated methods rather than human interviewers.

Trump has performed better this year in online or automated polls than in ones conducted by actual people.

The GOP candidate's campaign have attributed this to 'undercover' voters who refrain from declaring their loyalties to traditional pollsters, but who could help the billionaire win on the actual day.

'We think there's a big hidden Trump vote in this country,' campaign manager Kellyanne Conway told NBC last week.

Libertarian candidate Gary Johnson is polling at 7 per cent and Green Party hopeful Jill Stein at 3 per cent, the Rasmussen poll showed.

In Fox's first four-way poll of the candidates, Clinton gets 41 per cent and Trump 39. Johnson is at 9 and Stein at 4.

Another poll released Thursday by USA Today/Suffolk University had Clinton 7 points ahead of Trump at 48 per cent to 41 - close to the 6-point lead she had in the survey's last iteration in mid-July.