Republican presidential candidate Donald Trump has pulled into an effective tie with Democratic rival Hillary Clinton, according to the latest Reuters/Ipsos national tracking poll.

Trump has managed to erase a substantial deficit as he consolidates support among his party's likely voters with just two months to go before election day.

The poll showed 40 per cent of likely voters supporting Trump and 39 per cent backing Clinton for the week of August 26 to September 1.

Clinton's support has dropped steadily in the weekly tracking poll since August 25, eliminating what had been a eight-point lead for her.

Trump's gains came as Republican support for their party's candidate jumped by six percentage points over the past two weeks, to about 78 percent.

That is still below the 85 per cent support Republican nominee Mitt Romney enjoyed in the summer of 2012, but the improvement helps explain Trump's rise in the poll.

The candidates appear to be neck and neck after Trump made gains over the last week. But anything could happen between now and November 8 when voters go to the poll

Republican presidential nominee Donald Trump has closed in on his rival, Hillary Clinton

Mr Trump trailed Mrs Clinton in all of the major polls throughout the week, but results released on Friday appear to indicate the presidential race may be much closer than once believed

The Reuters/Ipsos poll is conducted online in English in all 50 states. The latest poll surveyed 1,804 likely voters over the course of the week; it had a credibility interval, a measure of accuracy, of three per cent.

Different polls have produced widely different results over the course of the campaign.

In part that's because some, like Reuters/Ipsos, have attempted to measure the preferences of who's likely to vote, while others have surveyed the larger pool of all registered voters. And even those that survey likely voters have different ways of estimating who is likely to cast a ballot.

Polling aggregators, which calculate averages of major polls, have shown that Clinton's lead has been shrinking for the past few weeks.

Those averages put her advantage over Trump at between three and six percentage points. Some of the more recent individual polls, however, have the race even tighter.

Voters don't elect the American president directly, of course, but through the Electoral College, an assembly representing each of the 50 states and the District of Columbia based on the number of legislators they have in Congress.

As of last Friday, the separate Reuters/Ipsos States of the Nation polling project estimated Clinton was on track to win the Electoral College, by about 332 votes to 206. Those numbers were scheduled to be updated later Friday.

In recent weeks, Clinton has come under renewed criticism over her handling of classified information while serving as U.S. secretary of state, and her family's charitable foundation has come under fresh scrutiny for the donations it accepted while Clinton served in the Obama administration. Meanwhile, Clinton hasn't been campaigning as actively as Trump.

Trump, meanwhile, has reshuffled his campaign leadership and sought to broaden his appeal to moderate Republicans and minorities.

He recently suggested that he would be a better president than Clinton for African Americans, and has taken steps, including a meeting this week with Mexican President Enrique Pena Nieto, to reach out to immigrants. It remains to be seen whether those efforts will click.

Mr Trump’s favor among Republicans has also bounced six percentage points to 78 per cent over the past two weeks, despite staff shake ups and his wavering stance on immigration

Clinton has led Trump through most of the campaign for the November election, though neither candidate appears to have inspired America.

In the latest Reuters/Ipsos poll, more than 20 per cent of likely voters opted for a choice other than the two major nominees, whether an alternative candidate, 'would not vote' or 'unsure.'

That figure is significantly higher than the 10 per cent to 14 per cent of respondents who answered similarly at this point in the 2012 campaign. Both President Barack Obama and Republican rival Mitt Romney enjoyed substantially stronger support at this point in the summer of 2012 than either Trump or Clinton does now.

And while Trump has consolidated his support among Republicans, likely voters are expressing an increasingly sour view of Clinton: The share of likely voters with an unfavorable view of the former secretary of state has grown to 57 per cent, compared with Trump's 54 per cent, her worst showing on that metric in a month.

Larry Sabato, director of the University of Virginia Center for Politics, said he remains convinced Clinton is ahead, somewhere in the range seen among the polling aggregators.

'There has been a closing that's completely natural,' Sabato said. 'Every four years, you have two national party conventions that produce a bounce of varying sizes. Clinton got a substantial bounce this year that lasted for a full month. It's usually gone around Labor Day, and by then we'll be where we should be, which is right around four to five points' for Clinton.