Clinton, Trump arrive at debate site for historic showdown

Game on.

With a tight election on the line, Republican Donald Trump and Democrat Hillary Clinton have arrived at Hofstra University in New York for their first presidential debate, a battle 18 months in the making that is one of 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.