Debates don't often change presidential races. Is 2016 the exception?

A debate sign hangs on a wall outside the media center setup for the first U.S. presidential debate at Hofstra University. Photo / AFP

Tomorrow's debate between Donald Trump and Hillary Clinton is the most anticipated event of a presidential campaign filled with remarkable and revolting moments.

But if the debate season, when it's all over, has changed the trajectory of the race, it will be a surprise. Amid all the sound and fury, changes now seem to come in inches.

The first debate at Hofstra University should be great theatre.

Trump, an unguided missile, will be seeking to project a calmer demeanour and a command of the facts enough at least to reassure voters who doubt his capacity to serve.

Clinton, a studious and well-prepared debater whose expertise on many issues is deep, will be looking to avoid the weeds, offer a bigger and more affirmative vision and mostly try to prove that her rival is unfit to occupy the Oval Office.

Trump will not be tethered to the teleprompter that has provided a modicum of discipline to his candidacy over the past few weeks and therefore will be the less predictable. He has not done a one-on-one debate, as she has, nor has he faced a format that, in theory at least, is designed to force the candidates to speak at greater length on the issues.

Debating skills have little to do with what makes a good president. President Barack Obama didn't lose the first debate to Mitt Romney in 2012 because he didn't know the issues. It was all about demeanour, energy and engagement. Debates are the ultimate show time in presidential campaigns. Trump is the acknowledged master of reality TV.

Yesterday the debate theatre was ramped up as Gennifer Flowers, who revealed a sexual relationship with Bill Clinton in the 1990s, said she would attend as Trump's guest.

Flowers's assistant confirmed the decision to BuzzFeed and Flowers herself told the New York Times in a text message, "Yes, I will be there".

The decision was the latest play in a bizarre bit of gamesmanship between the Clinton and Trump campaigns over the debate. Clinton's camp confirmed that they would invite billionaire Mark Cuban, a Trump antagonist, to the debate. Yesterday Trump tweeted: "If dopey Mark Cuban of failed Benefactor fame wants to sit in the front row, perhaps I will put Gennifer Flowers right alongside of him!"

Flowers said during Bill Clinton's 1992 presidential campaign that the two of them had engaged in an affair over a dozen years. The allegation, which Clinton denied at the time, threatened to ruin his campaign. In 1998, Bill Clinton acknowledged under oath having a sexual encounter with Flowers, though he disputed details of her account.

Clinton's team has suggested she is preparing for any eventuality from Trump, from the bombastic and sarcastic candidate who hectored his rivals during the Republican debates to the throttled back nominee of late whose goal will be to pin on Clinton anything that has gone wrong during Obama's eight years in office and convince the convincible that they really should want a change in direction.

Clinton's team is publicly demanding that moderator Lester Holt become a real-time fact-checker to a candidate - Trump - whose relationship to the truth has been so loose that he has earned a record number of Pinocchios and pants-on-fires from the media and other fact-checking operations.

What's crucial is, at this stage, Trump is not leading in states that have been consistently in the Democrats' column in recent elections. He is trailing in Pennsylvania, Michigan and Wisconsin, for example, all states where the demographics - older, whiter electorates with significant numbers of voters without college degrees - help him.

That's the context for the debate, a small Clinton lead nationally and several competitive state races with the map still more favourable to her. Can Trump change that - and defy history - with his performances in the debates? Can he move the white voters he needs to win? That's why tomorrow's audience could be huge.