US Presidential candidates Hillary Clinton and Donald Trump are preparing for what may turn out to be the most important 90 minutes of American political history, set to unfold on Monday.

Republican Trump and Democrat Clinton will face off for the first time in the first of three scheduled presidential debates on Monday at the Hofstra University on Long Island, New York.

According to analysts, this debate could turn out to be the most watched events in television history with an audience expected to exceed 100 million. It could beat a record set in 1980 when 80 million Americans watched Jimmy Carter and Ronald Reagan battle it out, CBS News reported.

Trump has began preparations in earnest on Friday when he finally broke from his hectic campaign schedule for a day without any public or private events so he could focus on his presentation for the 90-minute event that will begin at 9 p.m., with no commercial interruptions, the Guardian reported.

Clinton, in contrast, has spent most of this week with close aides at her Dutch colonial home in Chappaqua, a hamlet north of New York city.

According to sources, Trump has been watching videos of Clinton's best and worst debate moments, looking for her vulnerabilities, the New York Times said.

The analysts have said that the two candidates were taking vastly different approaches and their divergent strategies revealed how the two and their campaigns see the race, their strengths and their opponents' weaknesses.

Clinton's campaign has revealed that advisors to the former first lady were hurling a host of 'Trumpian' assaults and counter attacks at her to test her responses and adjust them as needed.

Clinton is eager to play offense and try to get under his skin, by doing things like calling him "Donald" and questioning his net worth, the New York Times quoted campaign officials as saying.

"I'll tell you 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 bigotry that we've seen coming from my opponent," she said during a radio interview earlier this week.

Trump's campaign does not want to fill his head with facts and figures, but instead, they want him to practice staying focused on bigger themes, such as jobs, terrorism, protecting the homeland and closing borders, rather than picking fights on side issues.

Working on her strategy, Clinton and her advisers have written out dozens of answers, and has tested some attack lines on the campaign trail to see what might work in the debate.

If Trump expresses admiration for President Vladimir V. Putin of Russia, she is prepared to evoke the hero of the Republican Party: "What would Ronald Reagan say about a Republican nominee who attacks American generals and heaps praise on Russia's president?" as she recently said.

Trump's main strategy during debates is often to attack and insult opponents, the analysts said.

He may tend to belittle Clinton to the point of rudeness when he is speaking off the cuff, such as by making fun of her for recuperating from her recent pneumonia, the New York Times noted.

However, Trump has promised a clean fight in which he will treat Clinton with "respect" and not bring up her husband Bill's past sexual indiscretions, the Guardian reported.

"I don't think I'm looking to do that," he told Fox News. "I don't know what I'm going to do exactly. It depends on what level she hits you with, if she's fair, if it's unfair, but certainly I'm not looking to do that."

Clinton is expected to conduct at least one mock debate over the weekend, whereas Trump will not do a full-length mock debate, according to their campaign officials.

The aim of these debates is to get a clear-cut understanding of how do the candidates differ from each other on all the issues that will be thrown at them.

The debate will be divided into six 15-minute time segments that begin with with a question, NBC News reported.

Each candidate gets two minutes to respond to the question, and then they can respond to each other. Whatever time is left the moderator can use to ask more questions or allow more debate.

Moderators can extend sections to make sure both candidates get equal speaking time.

The vice presidential debate is similar, but there's nine sections, each ten minutes long.

The moderator for Monday's debate is Lester Holt, a journalist with a decades-long career, first at CBS, and since 2000 at NBC News.

This debate will be followed by Vice-Presidential Debate at Longwood University, Farmville, Virginia between Democrat's Tim Kaine and Republican Mike Pence on October 4.

The second Presidential Debate will be at Washington University in St. Louis, Missouri on October 9. The third and final Presidential Debate will be at the University of Las Vegas, Nevada, on October 19.

Just like the election, Clinton is seen by some strategists as having a slight edge in the debate, but it is really not clear which way it will go. The latest NBC/ Wall Street Journal poll gives Clinton a 6 point lead, 43 per cent to Trump's 37 per cent, among likely voters.