COPENHAGEN, Denmark - The tense and tight battle to host the 2016 Olympics sped toward its climax with Chicago, backed by President Barack Obama's star power, the bookmakers' favourite Thursday after a long, emotional and at times acrimonious campaign.

The stakes are huge for all four cities -- Chicago, Rio de Janeiro, Tokyo, Madrid -- and for the International Olympic Committee. It faces the choice of sending the games to uncharted territory in South America, appealing to those committee members who believe that the Olympics should touch all corners of the globe, or opting in tough economic times for more familiar and perhaps more lucrative ground in Europe, the United States and Asia.

The winner gets the international prestige of staging the world's biggest sports extravaganza and billions of dollars in potential investment. The losers will rue what might have been.

Years of preparations and lobbying by the four candidates will come down to 30 minutes of voting by the IOC's members on Friday afternoon, when they will eliminate losers in successive rounds of secret balloting until one city receives a majority.

The IOC has 106 members, but president Jacques Rogge doesn't vote. As long as their cities are still in the race, members from Brazil, the United States, Spain and Japan will also be ineligible to vote.

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.

The round-by-round system makes predicting a winner perilous, because the outcome depends on how members shift their votes after their favourite candidates are eliminated. If Madrid is ejected first, would its supporters swing behind Rio, Tokyo or Chicago, and by how much in each case? And if Tokyo goes out next, where will its backers go? The variables are such that any city could conceivably go out -- even if Chicago and Rio have long been seen as the front-runners.

Of the IOC members eligible to vote tomorrow, two are Canadian.

Neither Richard Pound nor Beckie Scott would divulge which way they're leaning, but both are approaching the vote a bit dirrently.

Pound said he's looking for which city poses the least risk to the Olympic movement, while Scott is focused on the services the host would give to the athletes.

From the standpoint of Canadian Olympic officials, Chicago would be the best choice, as its proximity would make training and preapring Canada's athletes easier.

But, the choice of Chicago may effect Toronto's bid for the Pan American Games.

If South America is spurned by the Olympics, voters for the 2015 Pan Ams might be eager to place that event in either Lima or Bogota, the two cities running against Chicago.

Teams from all three are in Copenhagen this week, with the Canadian group including Ontario Premier Dalton McGuinty.

Vancouver Olympic organizers also have a role to play, albeit a small one.

They'll present their final official report to the IOC on Oct. 7, which will contain up-to-date review of how plans are shaping up for the 2010 Games.

According to British bookmakers, Chicago was pulling away in the last hours as the clear but not overwhelming favourite over Rio, with Madrid and Tokyo lagging.

"But don't forget that Paris was the favourite to beat London the last time," said Graham Sharpe, spokesman for betting agency William Hill. "The favourite doesn't always win."

All four cities were making their final pushes for support.

Brazilian President Luiz Inacio Lula da Silva insisted that Rio is ready "body and soul" if picked and he borrowed Obama's catchphrase, saying: "We want to overcome and show the world that yes we can."

Japan's new Prime Minister, Yukio Hatoyama, was arriving belatedly to boost Tokyo's flagging bid. The Japanese city's bid team wasn't sure if Hatoyama would actually have time for the important process of meeting and lobbying IOC members -- as Silva and first lady Michelle Obama have.

Obama will be joined Friday by her husband. President Obama is jetting in for a few hours to lend his charisma and appeal as the first black U.S. president to Chicago's final presentation to the IOC.

All four cities get 45 minutes to present their cases Friday before the vote, with follow-up questions. Some undecided IOC members are waiting to see how they perform before making their choice -- adding further uncertainty to the outcome.


With files from Canadian Press reporter Stephanie Levitz