Democratic presidential nominee Hillary Clinton's lead over Republican rival Donald Trump increased to more than seven percentage points in a Reuters/Ipsos poll, from less than 3 points last week.
The shift came as Trump struggled to reset his campaign following a stretch of controversies.
About 42 per cent of likely voters favoured Clinton and about 35 per cent preferred Trump, according to the August 4-8 online poll of 1152 likely voters, which had a credibility interval of plus or minus three percentage points.
The others would either pick another candidate, would not vote, or "don't know/refused".
The results reflected a decline in support for Trump, rather than a boost for Clinton: In last Thursday's poll, 42 per cent of likely voters favoured Clinton and about 39 per cent favoured Trump.
Related reading
Trump mocks Clinton over 'short-circuit' comment
Among registered voters over the same period, Clinton held a lead of nearly 13 percentage points, up from about five percentage points on Thursday, according to the poll.
The five-day survey concluded on a mixed day for the Trump campaign.
After squabbles last week with party leaders and the parents of a Muslim American soldier killed in Iraq, Trump sought to turn the page with a speech outlining an economic platform of tax breaks and regulatory rollbacks.
But in what was surely unwelcome news for Trump's campaign, 50 heavyweight Republican national security officials, in a letter published on Monday, said that Trump would be "the most reckless president in American history".
Trump hit back, saying the signatories "deserve the blame for making the world such a dangerous place".
Trump faced more dissent within his party on Monday. A former CIA officer, Evan McMullin, announced he would run as an independent alternative to Trump for conservative Republicans, and Republican Senator Susan Collins said she would not vote for Trump.
In a separate Reuters/Ipsos survey that gave respondents the option to choose from Clinton, Trump, Libertarian candidate Gary Johnson and Green Party candidate Jill Stein, Clinton leads Trump by about six percentage points.
Of the alternative party candidates, Johnson came in third with nearly eight percentage points, up from six points on Thursday.
Related reading
Trump blasts Clinton on TPP stance
