Hillary Clinton is leading Donald Trump by six points, a Reuters/Ipsos poll showed, as the candidates prepare to do battle at the second presidential debate.

Clinton sits comfortably ahead at 42 points, against Trump at 36. Libertarian Gary Johnson gets 8 per cent while the Green Party's Jill Stein gets a 2 per cent share.

The poll, published Wednesday, was conducted online before the vice presidential debate on Tuesday night, which a snap CNN survey indicated that Trump's running mate Mike Pence had won.

Donald Trump is trailing Hillary Clinton by six points, according to a Reuters/Ipsos poll

Clinton is enjoying her strongest-ever week, according to the RealClearPolitics polling average

Another poll, conducted Monday-Wednesday by Rasmussen and published Thursday, had Trump ahead of Clinton at 43 points to 41.

The phone and online survey has a margin of error of 2.5 points and was partially done after the VP showdown.

Gravis research published Wednesday had them both level-pegging at 44 points each.

Trump and Clinton will go head-to-head once again in the second presidential debate on Sunday night.

The RealClearPolitics poll of polls shows Clinton ahead by just over three points, at 43.9 to Trump's 40.7. Johnson gets 7.1.

Mike Pence (left) is thought to have come out ahead of Clinton's running mate Tim Kaine (right)

Clinton's showing has surged this week to her strongest since RealClearPolitics began its poll of polls in mid-May.

On Tuesday she recorded her highest-ever ratings, at 44.3 - even higher than the 44.0 which she was enjoying during a post-Democratic convention bump in early August.

Those ratings came after she successfully turned the spotlight on Trump following the first presidential debate. The Republican spent a week mired in controversy over his alleged mistreatment of former beauty queen Alicia Machado.

Clinton is still viewed unfavorably by 53.3 per cent of voters - but that's still a better rating than Trump, who gets 57.6 per cent, according to a RealClearPolitics average.