The latest polls show that the race is very close race. So with only 34 days until the election, Wednesday's debate is an important one for gaining the support of undecided voters.
ABC7's Charles Thomas is in Denver and provides a preview of the debate.
Throughout the morning and afternoon, the spin doctors for both campaigns made their pre-debate rounds.
"This is a very close race. And there are millions of Americans who haven't made up their minds yet who they're gonna vote for. There are other Americans that maybe are leaning one way or the other and could be persuaded," said Sen. Marco Rubio, (R) Florida.
Air Force One landed Wednesday afternoon in Denver, where Governor Romney has spent the past several days rehearsing for the big night. His adviser characterized the Republican nominee as the underdog.
"To use a baseball analogy, we're going up against a Cy Young winner here in the president who's an extremely good debater, so you have to get a lot of batting practice," said Kevin Madden, Romney adviser.
Meanwhile, the president's campaign dismissed as desperation a conservative website's overnight re-release of a 2007 video showing then-Senator Obama before a predominantly African-American audience in Virginia using a different speaking style.
"People will make up their mind on their own on whether it should factor into their vote," said Madden.
"That was a transparent attempt by Mitt Romney's allies to change the subject from that tape in which he wrote off half the population so they wouldn't take personal responsibility for their lives," said Ben LaBolt, Obama campaign spokesperson.
As debate preps continued so did the barrage of television and Internet ads targeting Colorado, one of the half dozen or so battleground states. Political experts predict the Obama and Romney campaigns could spend over $40 million here trying to convince undecided voters, estimated to be less than 5 percent of the electorate.
"Yeah, they're going to be spending a lot of money per voter. But given how close this race already is, we're really just talking about only a few percentage points difference between the candidates. The winner will probably see that as money well spent," said Prof. Seth Masket, University of Denver.
The debate will last 90 minutes in six 15-minute segments. An estimated 50 million people are expected to watch the debate.