Democratic presidential nominee Hillary Clinton leads Republican rival Donald Trump by 12 percentage points among likely voters, her strongest showing this month, according to a Reuters/Ipsos opinion poll released on Tuesday.

The Aug. 18-22 poll showed that 45 per cent of voters supported Clinton, while 33 per cent backed Trump ahead of the Nov. 8 election.

Clinton, the former U.S. secretary of state, has led Trump, a New York businessman, throughout most of the 2016 campaign. But her latest lead represents a stronger level of support than polls indicated over the past few weeks. Earlier in August, Clinton's lead over Trump ranged from 3 to 9 per centage points in the poll.

The poll also found that about 22 per cent of likely voters would not pick either candidate. That lack of support is high compared with how people responded to the poll during the 2012 presidential election between Democratic President Barack Obama and Republican Mitt Romney.

"Those who are wavering right now are just as likely to be thinking about supporting a third-party candidate instead, and not between Clinton and Trump," said Tom Smith, who directs the Center for the Study of Politics and Society at the University of Chicago.

During the latest polling, Clinton faced renewed scrutiny about her handling of classified emails while serving as secretary of state from 2009 to 2013, and Trump's campaign chief, Paul Manafort, resigned after a reshuffle of the candidate's campaign leadership team.

Clinton held a smaller lead in a separate four-way poll that included Libertarian nominee Gary Johnson and Jill Stein of the Green Party. Among likely voters, 41 per cent supported Clinton, while 33 per cent backed Trump. Johnson was backed by 7 per cent and Stein by 2 per cent.

The Reuters/Ipsos poll was conducted online in English in all 50 states. Both presidential polls included 1,115 respondents and had a credibility interval, a measure of accuracy, of 3 percentage points. – Reuters