Hillary Clinton is set to become the first woman presidential nominee of a major US party.

The day's star turn in Philadelphia will be the traditional warm spousal endorsement: with the twist that it will be former President Bill Clinton making the case for his wife to beat Republican Donald Trump.

Bill Clinton, president from 1993-2001, draws the admiration of many in the party, but carries some liabilities.

Liberals, including Bernie Sanders supporters, have long been critical of some of his trademark economic policies, such as the NAFTA free trade agreement with Mexico and Canada, and Wall Street deregulation.

At his best, Bill Clinton has been known as one of the most powerful political orators in the country: he made an emotional case in 2012 for the re-election of President Barack Obama.

In Tuesday's prime-time address he will be capping a day Democrats want to use to showcase Hillary Clinton's achievements on issues such as women and families, healthcare and as a former secretary of state.

Hillary Clinton, 68, will be watching him speak from their home in Chappaqua, New York, campaign officials said.

Opinion polls show Clinton as deeply unpopular with some Americans. Recent polls have given her "unfavourable" ratings that average 55.4 per cent, according to the RealClearPolitics website.

Trump has a similarly poor number, with "unfavourable" ratings averaging 56.9 per cent.

She is still disliked enough by some Sanders supporters for ill feelings to have spilled onto the floor of the convention on Monday, with some delegates booing her name.