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Donald Trump and Hillary Clinton will go head-to-head in the third and final presidential debate in Las Vegas today.

With 18 days left until America heads to the polls, there is plenty on the line when the candidates take to the stage.

Here’s what you need to know before it all starts.

When?

The debate begins at noon AEDT and will run for 90 minutes. The venue this time is the University of Nevada in Las Vegas.

How is this debate different?

There won’t be any questions from an audience — and the lecterns will be back.

This debate will be more traditional after the town hall-style of the last one.

Fox News Sunday host Chris Wallace will moderate alone. Photo: Getty.

Fox News’s Chris Wallace will be the sole moderator.

Topics the candidates will cover include:

• Debt and entitlements
• Immigration
• The economy
• The Supreme Court
• Foreign hot spots
• Fitness to be president

Worth noting is that these topics are “subject to possible changes because of news developments”, according to the Commission on Presidential Debates.

What does Trump need to do?

Amid reports Republicans are distancing themselves from their own nominee in an attempt to save their seats in Congress, Mr Trump needs to breathe life into his faltering campaign.

Time is running out for him to make up ground on Mrs Clinton, who has been gaining ground in the polls ever since the first debate.

The ABC’s Washington DC bureau chief, Zoe Daniel, said there was speculation Mr Trump would come out with a “scorched earth” policy, against the wishes of many of his closest advisers.

“What he has been told to do by strategic advisers is to take a more presidential approach.”

“Even his wife said in an interview 48 hours ago that he should be calm, he should be himself but he should talk to the policies and issues that the American people care about.”

What does Clinton need to do?

Mrs Clinton has been keeping a very low profile over the past week, largely staying out of the way as Mr Trump feuded with pretty much everyone.

That’s not by accident, according to Daniel.

“Perhaps it is a deliberate target strategy but she is being advised to appear presidential and rise above whatever Donald Trump might throw at her tonight,” she said.

“Trying to make her appear more appealing to the American people. That is where she needs to be going tonight.”