US President Barack Obama and Republican challenger Mitt Romney are sharing the stage for the first time in a high-stakes debate with the power to reshape the race for the White House.
The showdown at the University of Denver is critical for Mr Romney, a former Massachusetts governor. With five weeks to go before the election, polls show Mr Obama leading in the most important states in what is a state-by-state vote that decides the presidency. Still, the race remains tight and the three debates this month give Mr Romney an opportunity to shift the momentum, taking on Mr Obama before a television audience of millions.
This debate might be the most important of the three, with its focus on domestic issues that have dominated the race. Mr Romney was bound to hone in on the weak US economic recovery, arguing that Mr Obama's policies have failed and he does not deserve another term. Republicans say that Mr Romney, as a Washington outsider and successful businessman, knows how to create jobs and lift the economy.
Mr Obama argues that he prevented a meltdown after inheriting an economy in freefall from the Republican administration of George W Bush. He says Mr Romney would reinstate Bush-era policies that led to the financial crisis and help rich people, while hurting the poor and middle class. Either Mr Obama or moderator Jim Lehrer seemed likely to press Mr Romney about secretly recorded comments in which he said 47% of Americans view themselves as victims who depend on the government and refuse to take responsibility for their lives.
Much attention will be given to the style and body language of the two candidates. Mr Romney often comes across as stiff and distant, while Mr Obama is seen as warmer and more empathetic with everyday Americans. Polls show that most people expect Mr Obama to outperform him at the debate. But the high expectations for Mr Obama could mean that a weak performance would be more damaging for him than for Mr Romney.
The spotlight in presidential debates tends to fall on the candidates' mannerisms, their gaffes or their ability to get off a sharply worded jab. But this debate also showcases a clear-cut difference in philosophies. Mr Romney and fellow Republicans see the federal government as too big, taxing Americans excessively, running up deficits and hindering job creation through unnecessary regulations. Mr Obama and his fellow Democrats see government as a potential force for good, providing the infrastructure and education needed in a dynamic economy and giving even poor Americans the opportunity to succeed.
Though Election Day is more than a month away, many Americans have already started casting ballots because some states allow early voting. That puts extra pressure on Mr Romney to come up with a showing strong enough to alter the course of the campaign.
In what has become an American political tradition, both campaigns have tried to lower expectations for their candidates' performances in the debate, lavishing praise on their rival's debating skills. And just as inevitably, they will declare their candidate the runaway winner just as soon as the debate ends - if not sooner.
Mr Romney took part in 19 debates during the campaign for the Republican primary early in the year. Mr Obama has not been on stage with a political opponent since his last face-to-face encounter with John McCain, his Republican rival in 2008.
The next two debates are on October 16 in New York and October 22 in Florida.