Chicago, Rio de Janeiro, Madrid and Tokyo - one by one - will be eliminated in a tense half-hour of successive rounds of secret balloting until one stands alone.
Shortly afterward, IOC president Jacques Rogge will break open a sealed envelope containing the winner's name and utter the magic words: "The games of the 31st Olympiad are awarded to the city of ..."
But on the eve of the vote, the IOC's 106 members were, as ever, keeping their thoughts to themselves. Only Tokyo seemed to have fallen out of the running. Otherwise, it was still too close to call between the beaches and bossa nova of Rio, the bustle and Lake Michigan waterfront of Chicago or the European elegance of Madrid. Everyone had reason to be hopeful, none reason to be sure.
"This is the big question, how will it go?" said Willi Kaltschmitt, an IOC member since 1988.
"There have been so many surprises in the past," he said. "It's very even at the moment. It all depends what happens in the first round."
As cities go out, loyalties will shift. That is where the contest will be won or lost. If Tokyo goes out first, will its supporters swing behind Rio, Madrid or Chicago and by how much for each? Could Madrid stun front-runners Chicago and Rio in the second round, knocking one of them out, with its seemingly solid core of backers?
The variables are such that any city could conceivably win or lose. A few votes either way could decide it. That is especially true this time, with all four cities seen as generally capable of holding the games. Some IOC veterans say there has been no closer contest in recent memory.
To prevent bribery, IOC members aren't allowed to visit the bidding cities - so they'll be deciding instead based on what they've read in committee reports. Some will go with their gut instincts, their emotions and personal interests.
Which is where President Barack Obama comes in, literally. He jets in Friday morning, for just five hours, to try to tip the outcome to Chicago. An Obama star turn could swing it - or possibly rebound against him if his adoptive hometown is knocked out. Coming for the day of the vote is a political risk, but so, too, was the risk that Rio might win if Obama didn't lend his charisma and inspirational example to Chicago's final push.
His election as the first black president in U.S. history resonated loudly in Europe, which has the most IOC voting members - 46.
His wife, Michelle Obama, has worked the room before him, wowing IOC members with her charm and smarts. The first lady flew in Wednesday. While hesitant to declare that Barack Obama's appearance could be decisive, IOC members acknowledged it was hotly anticipated.
"It is a very special moment," said Gerhard Heiberg, an IOC executive board member from Norway. "Let me listen to him and see what kind of vision he has for the games."
Talk show star Oprah Winfrey is part of Chicago's hard sell, too. She swept Hollywood-style into the marbled lobby of the IOC hotel on Thursday, turning heads and stopping every other step to shake a hand or pose for a photo with IOC members and their wives.
But Chicago is up against equally charismatic opposition in the shape of Brazilian President Luiz Inacio Lula da Silva. The bearded former union leader makes a compelling case for Rio. Foremost of those is that South America has never before held an Olympics and that the games shouldn't be the exclusive preserve of rich, developed countries. That is an argument the other cities can't use because their countries have all held the Olympics before. Like Mrs. Obama, Silva has been lobbying IOC members personally in Copenhagen, as has Spanish King Juan Carlos.
Rio appeals to IOC members who believe it's their duty to share the Olympic ideals and pursuit of sporting excellence with all corners of the globe. South America is also an untapped market for Olympic sponsors. The romantic appeal of Rio's beaches and mountains is strong. The soothing samba song "Mas que nada" - "Don't worry about it" - playing in the auditorium lent an exotic, dreamy air to a news conference that Silva held Thursday.
"For some countries, it is just one more sports event that they are going to organize," he said. "But for us, it is a unique and extraordinary thing."
He even borrowed Obama's catchphrase: "We want to overcome and show the world that yes we can."
The final throw of the dice for all four cities will be their 45-minute presentations of speeches and videos, followed by questions, that they will make in turn Friday before the vote. Kaltschmitt said he believes that 40 percent of his colleagues on the IOC were undecided going into Friday and were waiting for the presentations to make up their minds.
Added Heiberg: "We've already heard about the technical plans. I want something more. I want to know about the vision. The leaders have to lift it up. I want to know why they want the games."
It takes a simple majority for victory, although it is unlikely that any city will win outright in the first or second rounds.
Rogge doesn't vote and, as long as their cities haven't been eliminated, neither can members from Brazil, the United States, Spain and Japan.
That leaves 97 voters in the first round, with more in subsequent rounds. In the event of a two-city tie in the early rounds, a runoff is held between the cities. If there is a tie in the final round, Rogge can vote or ask the IOC executive board to break the deadlock.
British bookmakers had Chicago pulling away in the last hours as the clear but not overwhelming favorite.
"But don't forget that Paris was the favorite to beat London the last time," said Graham Sharpe, spokesman for betting agency William Hill. "The favorite doesn't always win." ---
AP National Writer Nancy Armour and AP Sports Writers Graham Dunbar in Copenhagen and Stephen Wilson in London contributed to this report.
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