Rio urged the IOC's members to be bold by taking the games to South America for the first time, arguing that the continent offers a new frontier for the Olympic movement and that the games should not be the preserve of rich, developed countries.
"It is a time to address this imbalance," Silva said. "It is time to light the Olympic cauldron in a tropical country."
Chicago was no less forceful. Obama used both his stature as a statesman and his personal life story for impact. He held out the prospect of a Chicago games helping reconnect the United States with the world after the presidency of his predecessor, George W. Bush.
He pledged that the "full force of the White House" would be applied so "visitors from all around the world feel welcome and will come away with a sense of the incredible diversity of the American people."
"Over the last several years, sometimes that fundamental truth about the United States has been lost," Obama said. "One of the legacies, I think, of this Olympic Games in Chicago would be a restoration of that understanding of what the United States is all about and the United States' recognition of how we are linked to the world."
The presentations that all four cities were making represented the finishing line after years of hard work, lobbying, planning and hopes. They had 45 minutes and follow-up questions to sway and wow undecided IOC members, of which there were many after a long, close and at times acrimonious race.
The 103 members who attended the meeting start voting electronically in a secret ballot at 5:10 p.m. (1510 GMT). The vote will take up to 30 minutes. Cities will be eliminated one-by-one until one secures a majority.
Chicago presented its case to the IOC first, with videos and speeches - capped by Obama's plea. A central theme of the presentation was the city's people - none of them more famous than Obama and his wife Michelle, who both flew to Copenhagen - in the U.S. leader's case for less than five hours.
With the IOC's members sitting silently before him, Obama explained how his family moved around a lot when he was a kid and "I never really had roots."
But in Chicago, he said, "I finally found a home."
Tokyo was next, presenting itself as the best city for the athletes, safe and environmentally pioneering.
"Tokyo will show the world how a major metropolis can flourish without detriment to the environment," Japanese Prime Minister Yukio Hatoyama said.
Rio played up the wow factor of its fabulous scenery, with computer-generated bird's eye images of how venues would be spread across the city, with sailing in the shadow of Sugar Loaf mountain and volleyball on Copacabana beach. The governor of the central bank said Brazil's economic vibrancy should reassure IOC members, and the head of Rio state played down concerns over security.
But Rio's biggest selling point was that the IOC could ignore South America no longer.
"When you push the button today, you have the chance to inspire a new continent, make Olympic history," said Rio bid president Carlos Nuzman, who is also an IOC member. "Vote Rio, and we offer a gateway to 180 million passionate young people in South America."
An uncomfortable moment for Chicago came when an IOC member from Pakistan, Syed Shahid Ali, noted that going through U.S. customs can be harrowing for foreigners.
Obama responded that he wanted a Chicago games to offer "a reminder that America at its best is open to the world."
His wife tugged at IOC members' heart strings by discussing her late father, who had multiple sclerosis. She recounted sitting on his lap, watching Olympians such as Carl Lewis and Nadia Comaneci compete, and how her father "taught me how to throw a ball and a mean right hook."
"My dad would have been so proud to witness these games in Chicago," she said.
The high drama on Friday will come when IOC president Jacques Rogge announces the name of the winner about an hour after the last votes are cast. He will break open a sealed envelope and declare which city has been awarded the games of the 31st Olympiad.
The winner gets huge prestige and billions of dollars in potential economic benefits, the losers just painful thoughts of what might have been.
Rogge doesn't vote and, as long as their cities haven't been eliminated, neither will members from Brazil, the United States, Spain and Japan. Three other members did not attend the session.
That left 95 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.
Ahead of the vote, only Tokyo seemed to have fallen out of the running. But 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.