The candidates will be speaking to a TV audience of tens of millions in one of those rare moments when a critical mass of Americans collectively fix their attention on one event. Fifty-two million people tuned in to the first debate four years ago, and 80 percent of the nation's adults reported watching at least a bit of the debates between Obama and Republican John McCain in 2008.
In a quadrennial pre-debate ritual, each campaign has worked overtime to raise expectations for the opponent while lowering the bar for its own candidate. The thinking is that it's better to exceed lukewarm expectations than to fail to perform at an anticipated level of great skill.
But both men are seasoned debaters: Obama has been here before, facing off with McCain in 2008. Romney hasn't gone one on one in a presidential debate, but he got plenty of practice thinking on his feet during 19 multicandidate debates held during the Republican primaries.
On a long day of debate prep -- Romney in Denver and Obama in Henderson, Nev., near Las Vegas -- both candidates tried to blow off some steam Tuesday. The president made a tourist's visit to nearby Hoover Dam, and Romney fit in a lunchtime outing to a Mexican grill for a burrito bowl.
The two candidates' biggest fans talked up their debating abilities in pre-debate interviews.
Michelle Obama told CNN she's like a nervous parent watching a child performing on the balance beam when her husband debates.
"I do tell him to have fun and relax and just be himself, because the truth is, if he's the Barack Obama the country has come to know and trust, he is going to do a great job," she said.
Ann Romney said her husband always looks around to find her in the debate audience and keeps a paper in front of him that says "Dad" -- to remind him to make his father proud.
As for her advice, Mrs. Romney told KMGH-TV in Denver that she tells her husband: "Sweetie, you had five boys. You learned to argue really well and make your points years ago. Just go do that."
Wednesday's format: The moderator, PBS newsman Jim Lehrer, will open each 15-minute segment with a question, and Obama and Romney each will have two minutes to answer. After that, it's up to Lehrer to keep the conversation going and to intervene if one candidate goes on too long.
Obama and Romney have a two-track mission with debate viewers: Motivate core supporters to turn out and vote -- at a time when early voting already is under way in many states -- and try to lock in some new supporters from among the small subset of viewers who haven't settled on a candidate or whose support for one man or the other is squishy.
The viewers who matter most live in the contested battleground states that will determine which candidate gets to 270 electoral votes on Nov. 6: Colorado, Florida, Iowa, Nevada, New Hampshire, North Carolina, Ohio, Virginia and, to a lesser extent, Wisconsin.
Recent national polls show the two candidates in a tight race among likely voters. But Obama has the advantage in many of the battlegrounds, including Colorado.
Romney and Obama debate again Oct. 16 in Hempstead, N.Y., and Oct. 22 in Boca Raton, Fla. Biden and Ryan have their lone debate on Oct. 11 in Danville, Ky.
Obama plans to use the first presidential debate as the hook for fundraisers and recruiting volunteers. Bill Clinton will be in Boston on Wednesday night for Obama, with donors paying $20,000 a person. New York Gov. Andrew Cuomo is headlining a New York fundraiser.
The Obama campaign plans more than 4,000 debate-watching events around the country. And Biden is scheduled to hold a live discussion with supporters that will be streamed online after the debate.The Romney camp planned 336 debate parties at restaurants, bars, grills, VFW halls and other sites concentrated in battleground states.