WASHINGTON (REUTERS) - Presidential candidate Hillary Clinton's support among Democrats has dropped to its lowest point since Reuters/Ipsos began polling on her chances of winning the party's nomination for the 2016 election almost three years ago.

But the former secretary of state still has a lead of more than 20 percentage points over her nearest rival, Vermont Senator Bernie Sanders, the online survey showed on Friday.

Clinton has suffered a steady erosion in the number of people in opinion polls who see her as trustworthy as controversy has grown over her use of a private email account when she served as America's top diplomat.

The Reuters/Ipsos survey, a rolling poll taken over the previous five days, put Clinton at 45 per cent, with Sanders at 25 per cent.

The lead is Clinton's smallest since Reuters/Ipsos began polling Democrats in late 2012 about who they want to see representing them at the November 2016 presidential election.

Vice-President Joe Biden, who is considering whether to challenge Clinton for the nomination, came in third at 16 per cent.

Biden was polling at around 10 per cent a month ago.

A total of 494 people who identified themselves as Democrats took part in the poll, which had a credibility interval of plus or minus 5.1 percentage points.

Clinton's lead is now slightly smaller than the gap between Republican front-runner Donald Trump and his nearest rival, former Arkansas Governor Mike Huckabee, in a Reuters/Ipsos survey of Republicans.

Trump, with 33 per cent, led Huckabee, with 12 per cent, on Friday.