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(CNN)It could be the greatest political show on Earth.

With a tight election on the line, Republican Donald Trump and Democrat Hillary Clinton will face off in just hours at their first presidential debate, a battle 18 months in the making that is emerging as the most hotly anticipated moment in modern US political history.
An audience rivaling that of the Super Bowl -- perhaps around 100 million Americans -- will be glued to televisions, smart phones and social media when the rivals rip off the gloves at 9 pm ET. The debate marks a rare shared experience for a country deeply divided along political lines and fragmented in the media they consume.
Suspense has been building for weeks, given the huge political stakes of an increasingly competitive election. And Trump's wild-card antics, which will test Clinton's fact-checking skills, mean no one can predict how the showdown at Hofstra University in New York will unfold.
The rivals spent Monday prepping for their big battle.
Clinton took part in mock debates with her tart-tongued former aide Philippe Reines playing Trump. In one practice debate, Reines assumed the character of the unpredictable Trump by praising Clinton for her role as a pioneer for women, campaign sources said.
The Republican nominee has watched videos of Clinton but his preparation has been less intense than his opponent's, in keeping with his more freewheeling style. He did not hold mock debates, for instance, with someone standing in for Clinton.
It also emerged on Monday that there is no written agreement setting out the terms for the debates between the campaigns. Such deals have often been in place in previous debates, governing everything from the heights of the podiums to the topics of the event and the time available for each question.

Monumental stakes

The stakes of the debate are monumental.
Clinton, the Democratic nominee, is clinging to a narrow lead in many national polls, but now has almost no margin of error in the battleground states that will decide who will take the oath of office as the 45th President in January. A CNN/ORC poll released Monday found Trump edging Clinton 42% to 41% in Colorado among likely voters in a four-way race. In Pennsylvania, the poll found Clinton in a virtual tie against Trump among likely voters at 45% to 44%.
The Democratic nominee's task is to knock Trump off balance and force him to stick to facts instead of the vague -- sometimes outrageous -- statements he made during the GOP primary debate.
"We want these candidates to be judged fairly," Clinton campaign manager Robby Mook to CNN's Jake Tapper on "The Lead." "Do they both have specific plans to make people's lives better? Do they both have a real command of the issues?"
Trump, meanwhile, faces the challenge of bringing his unconventional style to one of the most traditional venues of a presidential campaign. His outsider campaign represents a repudiation of US domestic and foreign policy and if the debate helps convince Americans to elect him, he will lead the nation on a sharply different course than the one President Barack Obama has charted for nearly eight years.
Indiana Gov. Mike Pence, Trump's running mate, said the debate is a chance for the candidates to make their case directly to voters.
"You know when those two candidates take the stage for the first time in the same place, no more media filters, no more parsing of words," Pence said at a town hall in Milford, New Hampshire. "The American people are going to be able to hear from two candidates and they're going to hear about two futures for this country."
The destiny of the Supreme Court is up for grabs and the GOP's control of the Senate is on a knife-edge in an election that has sparked fierce controversies about race, faith, gender and the nature of America itself.
But there's another factor that makes Monday's debate, the first of three scheduled clashes, so significant. Clinton and Trump happen to be two of the most famous people in the country -- if not the world -- and their triumphs and disasters over the past quarter-century have reverberated far wider than the political bubble, embedding them in the fabric of American life.
In fact, the glass-ceiling-shattering female icon and the real estate magnate and reality star-turned-unlikely politician have the potential to lift the debate out of the political realm into a national cultural moment. It could be the kind of event where everyone will remember where they were when they saw it unfold.
"We have never seen any kind of interest like this, and probably never will ever again," said Aaron Kall, director of debate at the University of Michigan and editor of a new book called "Debating the Donald."
The 70-year-old Republican nominee has shattered every rule and convention in politics with his stunning presidential campaign, so there is no reason to think he will suddenly moderate his freewheeling style with the whole world watching.
Clinton, in facing a rival so instinctive and unpredictable as Trump, faces an assignment no previous nominee has confronted in the 56 years since John F. Kennedy and Richard Nixon held the first televised presidential debate.
"It's a watershed," said David Cram Helwich, a debate specialist at the University of Minnesota, who has carried out an in-depth study of Trump's debate technique. "Trump is a candidate who has defied all conventions of how one approaches these debates. It's unpredictable since he does not feel bound by traditional presidential debate convention and he could do whatever it is that he wants to."
But Clinton, 68, will make her own history as the first woman leading her ticket into a presidential debate -- a factor that could shift the dynamics of the previously all-male showdown and present her tough-talking rival with his own challenges.
Not even Trump seems to know how he will behave.
"If she treats me with respect, I will treat her with respect," Trump told Fox News' Bill O'Reilly last week. "It really depends. People ask me that question, 'Oh you're going to go out there and do this and that.' I really don't know that. You're going to have to feel it out when you're out there."
Clinton has in the past shown herself adept at capitalizing on any perceived sexism during debates. And Trump's worst moments in the primary season debates came when he was challenged over his alpha male approach by powerful women like Carly Fiorina or Fox News anchor Megyn Kelly.
Trump and Clinton enter the showdown with clear goals.
The billionaire real estate developer and neophyte politician must quell doubts that he lacks the knowledge, empathy and temperament needed to serve in the world's most unforgiving job, the presidency of the United States.
Clinton's policy expertise and experience are not in question. But her character and honesty are. The ex-secretary of state needs to show a glimpse of humanity to connect with Americans in way she never has before. She particularly needs to woo millennials crucial to her White House hopes.
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