ONE foot wrong could change it all.

In the space of 90 minutes this week, a showdown between Donald Trump and Hillary Clinton will mark the most significant moment in the trajectory of the US election campaign so far.

On live TV, America’s billionaire real estate showman will finally debate the country’s most seasoned politician.

Debates in the history of US presidential elections have been game changers — moments in time when the White House is won and lost.

With Clinton losing ground in the polls, and Trump gaining, the stakes have never been higher.

Just 43 days out, both will try to convince America that they’re ready to govern. It’s set to be the most watched debate of all time — a title currently held by the 1980 showdown between Jimmy Carter and Ronald Reagan.

Body language, one liners, and confidence will be scrutinised on both sides, as well as the substance with which they answer questions.

And it’s anybody’s game.

Poll: Clinton Keeps Lead Over Trump Poll: Clinton Keeps Lead Over Trump

HILLARY BUNKERS DOWN FOR DEBATE

The significance of just 90 minutes in a campaign that lasts for longer than a year is not lost on either Trump or Clinton, with both seeking to harness momentum as the race enters it final stretch.

Clinton — a seasoned debater — has carved out more time in her schedule than her opponent to get ready for the showdown.

Clinton this week visited just two key states — a marked decrease in her usual schedule — so she could spend time locked away with her time in practice sessions.

Trump taunted her on social media.

Hillary Clinton is taking the day off again, she needs the rest. Sleep well Hillary - see you at the debate! — Donald J. Trump (@realDonaldTrump) September 20, 2016

He repeated the same line at a recent rally: “Where is Hillary today?” he asked supporters.

“They say she’s practising for the debate. Some people think she’s sleeping.”

The Clinton team has more to lose in the match up. She’s participated in more debates than any American politician in recent times, and so bears the weight of expectation.

Clinton speculated that she didn’t know “which Trump” would show up at the debate.

“Maybe he will try to be presidential and try to convey a gravity that he hasn’t done before or will he come in and try to insult and try to score some points,” she said at a New York fundraiser.

Her team has been preparing for both versions of the man who stands in her way to get to the White House.

Clinton’s ‘Funny or Die’ Bid for Millennials Clinton’s ‘Funny or Die’ Bid for Millennials

CNN reported that Clinton’s team has been hunkered down trying to come up with a “one line retort that will be memorable”.

Tony Abbott famously used this strategy against Kevin Rudd, declaring at the Brisbane’s people’s forum “Does the guy ever shut up?” in 2013.

In her series of private debate sessions behind locked doors, she’s had advice from husband Bill and has had staffers role playing Trump.

“I am going to do my very best to communicate as clearly and fearlessly as I can in the face of the insults and the attacks and the bullying and the bigotry that we have seen coming from my opponent,” Clinton told the Steve Harvey Radio show this week.

“I can take it … I can take that kind of stuff. I have been at this, I understand it is a contact sport.”

SHOWMAN VOWS TO BE RESPECTFUL

Trump is expected to bring trademark showmanship to the event, whether he brings the insults Clinton is prepping for or not.

Unlike Clinton, Trump has kept his busy campaign schedule this week, and has maintained publicly that he’s relaxed about the debate.

“I believe you can prep too much for these things,” he told The New York Times.

He and his friends reportedly gathered for a joke-writing session over cheeseburgers, according to Vanity Fair.

When he takes to the stage, it’s almost certain Trump will target key Clinton scandals — ranging from most recently hiding her pneumonia diagnosis to things like her secret email server and involvement in Benghazi.

Beyond that, Trump has also proven before that even Bill Clinton’s sexual history is considered fair game as far as he’s concerned.

Trump knows how to debate and win audiences — it was his dramatic performances at the early Republican primary debates last year that first set him on the path to rising in polls and knocking off his competitors.

Back then though, he used shock and awe tactics — taking on actor Rosie O’Donnell and Fox News host Megyn Kelly. That’s a different Trump to the one who wants to appear presidential.

Trump previewed the debate this week saying he would be “respectful” of Clinton.

“I’m going to be very respectful of her,” Trump said in a phone interview on Fox News.

“I think she deserves that and I’m going to be nice. And if she’s respectful of me, that’ll be nice. “We’ll have something that I think people will respect as a debate but we’ll see where it all goes. You really never know exactly how it’s going to turn out and that’s why we’re going to have a lot of people watching.”

Clinton’s team has described Trump as a tough opponent, in an attempt to allay expectations that she should annihilate him due to her experience.

“For all his lack of substance, Trump’s showmanship, as ex-reality TV star, makes him a formidable debate foe. He thrashed his rivals in GOP debates,” Brian Fallon, Clinton’s national press secretary, tweeted.

In an email to backers this week, Clinton said Monday’s debate is the most important she’s ever participated in.

THE GREAT DEBATE

The great tradition of the US presidential debate began in 1960, in a famous head to head between John F. Kennedy and Richard Nixon.

The showdown was seen in many quarters to help Kennedy across the line.

Kennedy was sharp, well-spoken and polished; Nixon rolled in with five o’clock shadow, and was uncomfortable on stage. He had the flu, and was wiping his face with a handkerchief as he sweated on camera.

Reports at the time said it was that polished image on screen presented by Kennedy that made him the victor.

After politicians saw the damage a bad debate performance could do, it was then almost another 20 years before presidential candidates would go on TV for another debate.

Gerald Ford debated Jimmy Carter in 1976, where the incumbent president bungled a claim about Soviet rule in Eastern Europe.

Ford slid in the polls after the debate.

Fast-forward and there was “wristwatch-gate” of 1992. George HW Bush, debating Bill Clinton looked at his watch — a seemingly simple gesture that turned out to work against him.

It was reported at the time that to those watching, it was as if he Bush somewhere else to be and lacked interest, while Clinton was engaged with the people.

Even Al Gore in 2000 was niggled in the press for sighing audibly on camera in a head to head that saw his rival George W Bush gain ground.

HIGH STAKES

This week’s contest is a debate that comes at a crucial time for both sides, with Trump nipping at Clinton’s heels in the polling stakes.

Forecasts by poll analysts FiveThirtyEight found Clinton’s chance of winning in November were “near an all time low”.

The forecasters found that of 16 polls taken recently, 10 of those have swung towards Trump with an average of 2.8 percentage points. Just three have moved towards Clinton, with the rest steady.

“There’s … solid evidence that Clinton is still in decline” the site said.

Trump has carved out a narrow lead over Clinton in key battleground states — they are the places where the election will be decided — yet Clinton still holds a lead in the national polling.

A NBC/Wall Street Journal poll released this week have Clinton at a six-point lead on her competitor nationwide.

But in the battleground swing states, the story is different. A Fox News polls has Trump by five points in Ohio, five points in North Carolina, and three points in Nevada.

A Suffolk University poll found Trump ahead one point in Florida.

It comes as a new Reuters poll found the number of Americans who believe Clinton will be the president had plummeted 20 per cent.

DEBATE COULD RIVAL SUPERBOWL

When the debate kicks off in prime time Monday night in the US — 11am Tuesday morning in Australia — it’s expected 100 million Americans will tune in to the 90 minute head to head at Hofstra University in New York.

The format splits the debate into six segments, each 15 minutes long and beginning with moderated question.

The topics, finalised by the Commission on Presidential Debates and decided in advance, vary broadly from America’s direction, achieving prosperity and securing America.

Last election, just 70 million watched President Obama duke it out with Mitt Romney.

If Monday night’s ratings exceed expectations, the Trump/Clinton debate could rival the 2015 Super Bowl as the most watched TV program in US history — it recorded 114.4 million.

One thing is certain — the night will set the course for final six weeks in the race to the White House.