WASHINGTON, United States (AFP) — Billionaire Donald Trump extended his lead yet again atop the Republican presidential field, with front-running Democrat Hillary Clinton slipping and Vice President Joe Biden faring better than her against Republicans, poll results revealed Thursday.

Trump, the combative real estate mogul, leads the 16 other Republican candidates with 28 per cent support among registered voters nationwide, up from 20 per cent in a similar July 30 survey by Quinnipiac University.

Thursday’s lead marks the widest margin for any Republican so far in the election cycle, the survey said.

Retired neurosurgeon Ben Carson placed a distant second with 12 per cent, followed by former Florida governor Jeb Bush, Senator Ted Cruz and Senator Marco Rubio, each with seven percent.

Wisconsin Governor Scott Walker was next with six percent, and Ohio Governor John Kasich and businesswoman Carly Fiorina each registered five percent.

“Other GOP hopefuls seem to disappear. Trump proves you don’t have to be loved to be the leader,” said Quinnipiac poll assistant director Tim Malloy.

Trump’s abrasive style, including his caustic criticism of rivals, has led some observers to say he does not have the temperament to be president, but he has dominated the race essentially since he declared his candidacy in June.

On the Democratic side, Clinton’s support has shrunk, to 45 per cent now compared with 55 per cent on July 30, while her main rival Senator Bernie Sanders is polling at his highest level, 22 per cent.

Biden, who is not a declared candidate but is mulling jumping into the race, earned 18 per cent in the poll. Significantly, Biden “has the best appeal in general election matchups against top Republicans,” Malloy said.

Biden beats Trump by eight points, Bush by six points, and Rubio by three points.

Clinton beats them too but by smaller numbers: four points, two points and one point respectively.

Despite Trump’s Republican lead, he has the highest negative favorability numbers in the field, at 36 percent to 54 percent. Bush is underwater as well, at 32-41, while Clinton’s negative 39-51 matches her worst favorability rating ever.

Biden’s favorability rating was positive, at 48-39, while Rubio’s was 41-27 and Sanders 32-28.

Trump and Clinton are also seen as the least honest and trustworthy of the field.

Trump also tops the “no way” list, with 26 per cent of Republicans saying they definitely would not support him.

Clinton tops the “no way” list among Democrats, at 11 per cent.

The August 20-25 poll has a margin of error of plus or minus 2.5 percentage points.