The first presidential debate of the US general election is often the most treacherous - especially for the candidate who steps on stage with the presumed advantage.

Which is why Democratic nominee Hillary Clinton, the one in that position this time around, knows not to take anything for granted.

Tomorrow's 90-minute faceoff is projected to have the biggest audience ever with estimates that more than 100 million people will watch.

"You can't really win an election in a debate, but you can lose one," said Brett O'Donnell, a communications consultant with long experience coaching Republican presidential candidates.


"The first debate is the most important of all the debates, and it definitely has the most potential to harm."

Examples of first-debate stumbles are many.

The biggest pitfall is a blunder that confirms the misgivings that voters may already be harbouring.

A confused Ronald Reagan rambled in 1984, opening doubts about whether he had become too old to do the most important job in the world.

In 2000, Al Gore sighed and exaggerated. George W. Bush casually draped himself over the lectern in 2004 and peevishly quibbled on minor points.

As this year's candidates prepare to meet, Clinton is seen as the nominee best equipped to present herself as the more credible and appealing potential president.

The biggest unknown for Clinton and her strategists is which version of Trump will show up. It will be the first time he has appeared on a debate stage with only one other candidate, which means the spotlight will be harsher and more intense.

Trump has indicated he will approach the debate as he has pretty much everything else in his campaign - reactively, and by trusting his own instincts and impulses.

Donald Trump speaks during a campaign rally at the James L. Knight Center in Miami. Photo / AP
Donald Trump speaks during a campaign rally at the James L. Knight Center in Miami. Photo / AP