Presidential contenders Barack Obama and Mitt Romney are set to square off in their first face-to-face debate, an event that is expected to be viewed by as many as 60 million US citizens.

On Wednesday night, the two will spend 90 minutes at the University of Denver in Colorado sparring over domestic policy.

Although unemployment has remained high during Obama’s first term in office - over eight per cent for 43 straight months - many voters seem willing to accept his argument that he inherited a bad economy from his predecessor, George W Bush.

With opinion polls showing Romney, the former governor of Massachusetts, behind in the polls, the pressure will be on for him to deliver an outstanding performance that can shake up the race.

For Obama, Tuesday's debate, moderated by Jim Lehrer, exeucitve editor of the PBS Newshour, also falls on the same date as the twentieth anniversary of his wedding with Michelle Obama, the first lady.

'Knock it out of the park'

"Americans who are thinking about voting for Romney need to hear from him about how he would change the country for the better," said Republican strategist Ron Bonjean.

"They're leaning toward the devil they know, which is President Obama. Romney has to knock it out of the park by showing the contrast between himself and Obama."

Romney has recently been under fire for comments he made at a secretly recorded fundraising event, in which he said 47 per cent of US voters are dependent on government and unlikely to support him. "My job is not to worry about those people. I'll never convince them that they should take personal responsibility and care for their lives," he said .

Strong performances in the debates can have big effects on polling numbers: in 2004, Democratic presidential candidate John Kerry hammered George Bush on foreign policy, and temporarily erased Bush’s lead in national polls - though ended up losing the election.

Al Jazeera's Patty Culhane, reporting from Denver, said the two candidates "want to rally the people at home, the people who already support them. ... More importantly, though, there's about five per cent of voters who say they still haven't made up their mind."

With an expected television audience of between 50 and 60 million people, the debate is "an opportunity these candidates can't afford to miss".

The Commission on Presidential Debates, the non-partisan group organising the event, plans to introduce a new format this year. The debate will be divided into six discussion segments.

Peter Eyre, a senior adviser to the commission, said there will be "fewer questions but more extended discussion that dives into certain issues in detail".

No part in logistics

Eyre stressed that neither party, nor their candidates, had no part in the logistics of the debates.

"The campaigns and the candidates have really no input into how the set looks and feels, the formats, things like that. Those are decisions made by the Commission," he said.

Though there is a new format, Eyre says for the last two decades, the set for these debates has remained largely untouched.

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"We've been using this set behind me since really 1988 and obviously we've been improving it, enhancing over the years," Eyre said.

"But the set is designed by the Commission and that's really what we've been using since."

Despite their heated competition for the presidency, Obama and Romney have little personal relationship, and have rarely met in-person with one another.

Both Romney and Obama spent their time mostly in private on Tuesday, preparing for the debate.

The president was in Henderson, Nevada, near Las Vegas, while Romney was already in Denver.

Neither held public campaign events, but Obama took a break from preparation to visit nearby Hoover Dam, and Romney picked up lunch at a Chipotle Mexican Grill near his hotel.

Joe Biden and Paul Ryan will face one another in the sole vice presidential debate in Kentucky on October 11.