Democratic presidential hopeful Hillary Clinton posted an 11 percentage point lead over her Republican rival Donald Trump, according to a Reuters/Ipsos poll.
The June 24-28 poll showed that 45.3 percent of likely American voters support Ms Clinton while 34.1 percent support Mr Trump, and another 20.5 percent support neither.
Ms Clinton's lead was 14 points on Friday, though she has generally been widening her advantage over the New York real estate magnate since mid-May, when the two were nearly tied.
The former secretary of state, senator and first lady was bolstered in recent days by endorsements from members of the Republican establishment, including Henry Paulson, Treasury secretary under former President George W. Bush.
Her Democratic rival Bernie Sanders, a US senator from Vermont, said last week he would vote for her to stop Mr Trump even though his campaign said this was not a formal endorsement.
Ms Clinton is struggling to win over Mr Sanders supporters after a hard-fought campaign for the nomination. Critics have assailed her over her handling of emails and a 2012 attack on a US mission in Libya while secretary of state.
Mr Trump got a brief boost in the days after the June 12 mass shooting in Florida, coming within nine points of Ms Clinton as he fine-tuned a campaign promise to temporarily ban the entry of Muslim immigrants to shore up national security.
His level of support so far in June, however, lags behind what his predecessor, Republican White House hopeful Mitt Romney, received in the same period in 2012.
He has sparred with Republican leaders over his off-the-cuff rhetoric and lagged behind Ms Clinton's campaign organisation in both size and fundraising, worrying some of his allies.
The poll included 1099 likely voters and has a credibility interval, a measure of accuracy, of 3.4 percentage points.