Donald Trump and Hillary Clinton face off Monday in a presidential debate that could be the most-watched in US history, with the high-stakes showdown expected to attract up to 100 million viewers.

The two candidates continued preparing for their first debate until just before the showdown, their campaigns said.

If the number of viewers reaches 100 million, it would far surpass the 80 million who watched the 1980 presidential debate between Jimmy Carter and Ronald Reagan.

Anticipation has been building for the head-to-head encounter, which will last 90 minutes and feature questions about the direction of the country, the economy and national security.

The contest at Hofstra University on Long Island, New York is the first of three planned debates and is one of the last chances for the two candidates to sway voters ahead of the November 8 election.

Clinton, a Democrat whose political career spans four decades, is widely seen as the better debater. But Trump, the Republican who made his name in real estate and reality television, has a bombastic and unpredictable style that analysts say makes him a tricky opponent to go up against.

The media, political journalists in particular, commentators and viewers are expected to pay attention to both the verbal exchanges and the candidates' body language, with everything open to interpretation.

If the results of previous presidential debates are any indication, the contest will be less about who won and more about who lost. Trump, according to observers quoted in US media, has done a good job in setting expectations low. The political outsider could be viewed as the winner - or at least not the loser - if he doesn't make any big mistakes in the verbal contest with the politically experienced Clinton.

The stakes are high as both candidates attempt to improve their standing among voters. An opinion poll released Monday showed a dead heat at the national level, with both nominees receiving 46 per cent of likely voters in a Bloomberg survey.

But the presidential election is won state-by-state - especially so-called swing-states such as Ohio, Pennsylvania, North Carolina and Florida - and by that measure Clinton still has a slight advantage.

The candidates have invited some provocative guests to be in the audience. In the Trump camp will be a survivor of the 2012 attack on the US consulate in Benghazi, Libya, according to Fox News. Clinton was secretary of state at the time of the attack, which left four Americans, including the US ambassador to Libya, dead. Trump has criticized her for providing too little security ahead of the attack and said her response to it as inadequate.

For her part Clinton has invited billionaire investor Mark Cuban, a leading figure in the anti-Trump movement.

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