US President Barack Obama on Sunday set the table for his first debate with Mitt Romney, billing the clash as about security for hard working Americans, not who could fire off the best 'zingers'.

Obama heaped pressure on his foe, who is angling for a dramatic turnaround for his ailing campaign in Wednesday's showdown, before bunkering down in a resort in Nevada's rocky desert to shake off his mothballed debating skills.

"The media is speculating already on who is going to have the best zingers ... who's going to put the most points on the board," Obama told a floodlit rally of 11,000 people at dusk in a Hispanic suburb of Las Vegas.

"Governor Romney, he's a good debater ... I'm just okay," Obama said tongue-in-cheek, as he upped the stakes in the classic game of expectations setting that rival campaigns wage before big debates.

Obama sought to frame the face-off, the first of a trio of presidential debates, as a contrast between substance and style, and to paint himself as the champion of the hurting middle class.

"What I am most concerned about is having a serious discussion about what we need to do to keep the country going and restore security for hard working Americans," Obama said.

"That is what people are going to be listening for, that is the debate you deserve," the president told a crowd amped up by the Mexican rock band Mana.

Republican Romney and Democrat Obama will meet in Denver, Colorado on Wednesday night for the first of three debates crucial to shaping the remaining five weeks of the president's bid for a second White House term.

Obama currently leads the national race -- by five points in the latest Gallup daily tracking poll -- and in most key battlegrounds.

While the classic pre-debate spin requires campaigns to play down the chances of their man, gruff New Jersey Governor Chris Christie appeared not to get the memo from the Romney campaign.

"Wednesday night is the restart of this campaign and I think you're going to see those numbers start to move right back in the other direction," Christie said.

"This whole race is going to be turned upside down come Thursday morning."

The Obama campaign pounced on the comment, which will permit the president's camp to portray the former Massachusetts governor's performance on Wednesday as disappointing, whatever the outcome of the debate.

"What (Christie) said is what they've been saying for months, that they expect that after the debates will turn the race upside down," Obama spokeswoman Jen Psaki told reporters aboard Air Force One.

"We fully expect that that's what they're focused on."

Romney's vice-presidential pick Paul Ryan admitted to 'some missteps' after Romney's nightmare month in the White House race, and sought to moderate expectations, saying no debate would 'make or break' the campaign.

Obama is ''a very gifted speaker," Ryan said. "The man's been on the national stage for many years. He's an experienced debater. He's done these kinds of debates before. This is Mitt's first time on this kind of a stage."

Ryan also hammered Obama over the White House's shifting account of the assault on the US consulate in Benghazi.

"It's part of a bigger picture of the fact that the Obama foreign policy is unraveling literally before our eyes on our TV screens," Ryan said on Fox News Sunday revealing that Romney was planning a major foreign policy speech soon.

Initially, US officials said the September 11 assault in Benghazi was a spontaneous demonstration whipped by outrage over an anti-Muslim film made on US soil.

But they are now describing the incident as terrorism with possible links to Al-Qaeda, fueling Republican claims that the Obama administration mounted a cover-up to preserve the president's favorable ratings on national security.

Republican Senator John McCain said the administration's narrative did not pass the "smell test."

"It was either willful ignorance or abysmal intelligence to think that people come to spontaneous demonstrations with heavy weapons, mortars and the attack goes on for hours," he said.

Obama adviser David Plouffe said it was 'preposterous and really offensive' to suggest the government withheld information on Libya for political reasons.