Donald Trump is expected to try and "wing" his first head-to-head debate with Hillary Clinton on Monday after reportedly doing next to no preparation.

The populist Republican goes in to the first of a series of televised debates between four and six points behind the former Democrat secretary of state in national polls.

However, campaign sources stress the billionaire has focused on his stump speeches around the country rather than research the woman he calls "crooked Hillary".

Clinton, meanwhile, is said to have done hugely detailed research on Trump, who - in a sign of just how bitter the presidential elections are - she has characterised as a "habitual liar".

The Democrat's better preparation may not be crucial, suggested Richard Himelfarb, who teaches politics at Hofstra University in New York, the debate venue. "It's going to be decided on body language, tone and temperament," he told Fox News.