The two US presidential candidates are to face each other in their first TV debate, in what could be a defining moment of the election campaign.
Republican Donald Trump and Democrat Hillary Clinton will take to the stage in New York on Monday night.
The duel at Hofstra University could be the most watched debate in television history, with 100 million viewers.
There are 43 days until the election, with one poll on Monday suggesting some movement towards Mr Trump.
The poll, by Bloomberg, shows he holds a lead over his Democrat rival when third-party nominees are taken into account, and that they are tied in a head-to-head contest.
While other polls give Mrs Clinton a small lead, a joint poll between the Washington Post and ABC News on Sunday showed the two to be in a virtual dead-heat.
On Monday, Mr Trump’s campaign manager Kellyanne Conway told MSNBC that the “natural connective tissue he has with people” will be on show. “I can see that this man is ready for tonight,” she added.
Meanwhile, Mrs Clinton’s campaign manager Robby Mook told NBC’s “Today” show that their camp hoped Mrs Clinton would come out on top on “the fine points of policy,” but that they were concerned Mr Trump would get “the most improved award”.
“Donald Trump is an experienced reality show entertainer so he may decide this is a chance to show his chops,” he added.
The debate at 21:00 local time (01:00 GMT on Tuesday) will last 90 minutes and is being moderated by NBC news anchor Lester Holt.
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It is the most hotly anticipated event so far in a long election campaign, partly due to the contrasting styles of the two candidates.
As hotel developer and reality TV star Mr Trump marched to a stunning win in the Republican primaries against vastly more experienced political opponents, he hurled personal insults and made suggestive remarks on the debate stage.
Mrs Clinton, with decades of experience in politics, usually relies more on a firm and detailed policy grasp, but has problems portraying authenticity and spontaneity.
Observers predict the audience could be as high as that for the Super Bowl and surpass the 80 million who watched Jimmy Carter and Ronald Reagan debate in 1980.
Controversy has marked the debate build-up after Mr Trump said he might invite a woman who had an affair with Bill Clinton in the 1970s.
He tweeted on Saturday that he would perhaps ask Gennifer Flowers to sit in the debate audience, in response to Mrs Clinton having invited billionaire Trump critic Mark Cuban.
Ms Flowers initially said she would attend but Mr Trump's running mate, Mike Pence, said on Sunday she was not coming and the suggestion was not a serious one.
Three of the topics for the six segments of the debate have already been announced - America's Direction, Achieving Prosperity and Securing America - but three others, based on events in the news, will be asked during the debate.
In the past week both candidates have focused on the response to fatal police shootings of African-American men in Tulsa, Oklahoma, and Charlotte, North Carolina, as well as the ensuing protests.
What's at stake? Anthony Zurcher, BBC News, Washington
The debates are the last, best chance for Donald Trump and Hillary Clinton to make their case to the nation.
Neither candidate will be able to command the attention of the American public the way they will in the three upcoming presidential debates. And no debate will have as large an audience, or do as much to shape public perceptions of the candidates, as this first one.
There's a good chance Monday night's affair will break the record of 80 million Americans who watched incumbent Jimmy Carter face-off against Ronald Reagan.
In an election cycle that's measured in months and even years, this debate gives Americans a real, unscripted opportunity to see how the would-be presidents might handle the intense stresses of the Oval Office.
It's the mother of all job interviews.
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