A debate can’t win an election for you, but it can lose it for you.

Everything is heightened ahead of tonight's first presidential debate - the stakes are high, the viewership will be high, and for both candidates the nerves are likely high too, especially given that three polls in recent days show them locked in a dead heat (Washington Post-NBC, Morning Consult, and Bloomberg).

By his own admission Donald Trump had never debated before his run for the Presidency.

Hillary Clinton is an experienced debater: aside from this primary season where she debated Bernie Sanders head-to-head on several occasions, she also debated Barack Obama one-on-one 25 times in 2008.

Trump, on the other hand, first took part in debates this primary season, sharing the stage with various combinations from three to 16 opponents.

He rarely spoke for more than a minute or two at a time, and focused on standing out from the crowd rather than engaging in substantive policy matters.

So expectations from him are low - if he turns up and doesn’t lose his temper, and doesn’t expose his apparent shallow knowledge of key matters, he’ll be judged to have done well.

However, if pushed, he may struggle to expand on the details of some of his controversial policies.

From Clinton, much is expected. She’s expected to be smooth and polished and there is no question that she knows her facts and has thought-out policies.

But critically, she doesn’t really know what opponent will turn up tonight.

Will it be calm, softly speaking "presidential" Trump who has appeared of late?

Or will it be the shouting, doomsaying Trump, like the one who appeared at the Republican National Convention warning that the end was nigh unless he was elected?

Or will it be the Trump who insults virtually every group in society except white middle-aged men, and spouts generalities and falsehoods at will?

That Trump may be the hardest for Clinton to engage with - how do you debate someone when there is no accounting for what argument they will make next, and little basis to many arguments?

She will need to correct him and show her mettle, without coming across as the cleverest girl in class who is always right.

Donald Trump is likely to challenge her on the exact nature of her recent bout of pneumonia, and why she took so long to admit to it.

He has repeatedly labeled her "crooked" and "dishonest" and will try to draw that out on stage, to prove she has issues with truth and transparency, and paint himself as the "straight-up guy".

Then there is the issue of truth, exaggeration and just plain old falsehoods.

Clinton’s campaign aides said on Friday that they believed the role of fact-checking rested with the moderator and not with her.

She will, they said, correct the more egregious statements, but she does not want to waste her speaking time dealing with his mistruths, and would prefer to focus on her own policies.

The moderator on this occasion, NBC host Lester Holt, has not commented on whether he will fact-check, but the Commission on Presidential Debates has somewhat controversially said it doesn’t believe it’s up to the moderator to fact-check the candidates.

That raises again the role that the US media organisations, particularly the TV networks, have played in Trump’s success to date.

Tonight’s debate will be further testament to that, as an estimated 100 million viewers in the US (and countless others internationally) will tune in for what is set to be the most watched debate in US television history, and may even beat Superbowl TV audience figures.