After hundreds of campaign stops, US$500 million in mostly negative ads and countless tit-for-tat attacks, President Barack Obama and Mitt Romney go head to head this afternoon in their debut debate.
Both candidates completed walkthroughs of the venue at the University of Denver, hours before the first of three televised showdowns just 33 days before American voters decide their fates.
The debate begins at 2pm New Zealand time, and is being streamed live on YouTube.
Obama clings to a narrow lead in his bid to defy the omens sown by a stubbornly sluggish economic recovery and to become only the second Democrat since World War II to win a second term.
Republican Romney, down in almost all the key battleground states that will decide who wins the 270 electoral votes needed to win on November 6, seeks a sharp change of momentum in a race that seems to be slipping away from him.
The rivals will step up to podiums in the Rocky Mountain state of Colorado, at 7:00 pm local time to clash over the economy and other domestic issues.
But veteran anchor Jim Lehrer of PBS NewsHour, who will steer the debate for tens of millions of viewers at home, has leeway to bring up other subjects.
That means Obama, 51, could face a grilling on his administration's shifting account of the attack on the US consulate in Benghazi, Libya on September 11.
Romney, a multi-millionaire former venture capitalist, could come under scrutiny over his complex offshore tax arrangements, which Democrats have highlighted to press the case that he is indifferent to middle-class struggles.
The 65-year-old badly needs to reset the election narrative, after a video emerged of him branding 47 per cent of Americans as people who pay no taxes and see themselves as "victims" who depend on government handouts.
Obama and Romney, who have rarely met or spoken, have spent days in seclusion honing debate techniques, offensive parries and comebacks.
Both campaigns engaged in last-minute spin, talking up their opponents in order to make it tougher for them to live up to expectations.
"President Obama is a gifted speaker and won every debate in 2008. He will surely offer an impressively polished performance once again," Republican National Committee chairman Reince Priebus wrote in The Denver Post.
"But as we've learned the last four years, eloquence alone will not solve our nation's problems," he said.
With a record of high unemployment and massive debt, Obama "will make excuses (and) spend the debate dishonestly attacking governor Romney - exactly what he's been doing at campaign rallies and on the airwaves for months now."
And Republicans delighted in a verbal slip yesterday by Vice President Joe Biden, who said the middle class had been "buried" for the last four years.
Democrats said Biden was talking about how president George W. Bush's policies continued to hurt the middle class deep into Obama's term.
Romney's campaign released a Web video showing Biden's comments, followed by the simple line: "We couldn't have said it better ourselves."
The Obama camp countered with accusations that Romney has no specific plans to create jobs or move the country forward, "only tired repeats that will take us back," deputy campaign manager Stephanie Cutter wrote.
"Romney can use tonight's debate to fill in those details... or he can spend 90 minutes doing what he does best: attacking the president, distorting his own record and avoiding any and all details on his plans for this country."
Such spin will carry on during the debate itself through Romney's website at debates.mittromney.com and Obama's www.barackobama.com/debate.
Obama's site was trailing a "cheat sheet" outlining what the Democratic candidate contends that Romney will say to try and win favor.
Romney's site listed a set of "debate facts," noting that the United States had suffered a credit rating downgrade under Obama and said the national debt had risen by $5.4 trillion on his watch.
On Twitter, the battle was already underway between Republican accounts, including the official @MittRomney and @RomneyResponse, and Obama side @BarackObama and @TruthTeam2012.
Polls show race is tight
Several national polls released before the debate showed a tight race, with Obama ahead by a few points.
A new Wall Street Journal/NBC News poll gave Obama a lead among likely voters of 49-46 per cent, consistent with a RealClearPolitics poll average showing the graying US leader up by 3.5 percentage points.
A National Public Radio poll showed Obama leading 51-44 among likely voters nationwide and 50-44 in battleground states, while a Washington Post-ABC News poll gave Obama a larger 52-41 lead in swing states.