(CNN) -- Game on.
With a tight election on the line, Republican Donald Trump and Democrat Hillary Clinton will face off in less than an hour 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.
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%.
Nationally, CNN's Poll of Polls finds Clinton and Trump neck-and-neck 44%-42%.
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 Trump would be a "bold truth teller."
"My only advice to him is be yourself," Pence told Tapper.
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.
National cultural moment
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.
Both rivals know that presidential debates are not an academic exercise -- often the winner is not the candidate who shows the clearest command of the facts but the one whose presence, body language and personality come across as closest to the popular understanding of presidential gravitas and bearing.
They are both also making political points with their choice of guests for the big event.
Clinton has invited billionaire businessman and prominent Trump critic Mark Cuban; 9/11 survivor Lauren Manning; Maxine Outerbridge, who benefited from a children's health insurance program the Democratic nominee backed as first lady; Anastasia Somoza, a disability rights advocate; and Aleatha Williams, her longtime pen pal.
Trump is bringing two former members of the military brass who have backed his campaign, Gen. Michael Flynn and Gen. Keith Kellogg He also invited Karen Vaughn, who lost her Navy SEAL son in Afghanistan and Mark Geist, a survivor of the attacks on a US compound in Benghazi, Libya, in 2012.
Both candidates are also expected to bring close family members to the debate.
Monday's showdown is especially notable since Trump has never taken part in a one-on-one debate. The packed stage at the GOP primary debates often allowed him to bluster through critiques of his thin policy agenda and experience or to take time-outs while other candidates sparred.
But the concentrated spotlight at the 90-minute presidential debate -- with no commercial breaks -- makes it more difficult to get away with the vague, sweeping and untruthful statements with which he peppered the primary showdowns.
Still, the Clinton campaign is showing real signs of concern that Trump's method will simply blow up the format of the presidential debates -- and the way the media and audience evaluate who won.
"Donald Trump has pattern of repeating lies hoping no one will correct him," said Jennifer Palmieri, Clinton's communications director last week.
"Any candidate who tells this many lies clearly can't win the debate on the merits," she said. "His level of lying is unprecedented in American politics."
Trump's behavior, and frequent refuge in falsehoods, will also challenge the conventions of how debates are normally judged, placing intense pressure on moderator Lester Holt of NBC. The post-game spin will also help decide how this first of three debates influences the election.
"Historically, the debates have not overcome the fundamentals of the election," said Republican pollster Whit Ayers on CNN's new podcast "Party People." "That said, there have been debate moments that we can all remember that have galvanized the election and have led to one candidate doing substantially better."