COPENHAGEN — 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' favorite 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 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.

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

"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."

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 the city with the fewest votes in successive rounds of secret balloting until one city receives a majority.

The round-by-round system makes predicting a winner perilous, because the outcome depends on how members shift their votes after their favorite 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.

All four cities were making their final pushes for support.

Brazilian president Luiz Inacio Lula da Silva insisted 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 to Chicago's final presentation.

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.