Round by round the 100 or so IOC members make their selection, bumping the city with the least votes, then voting again until one city manages to win a majority: 50-percent plus one. "That" will be the city the IOC dances with until 2016.
"You watch these things and the IOC tends to back into its decisions by sort of eliminating the ones that seem to have the most risk attached," said Dick Pound, IOC member, Canada.
On its face, the make up of the International Olympic Committee seems stacked against a U.S. bid. There are only four IOC members from North America who will get a vote; ten from Central and South America; 44 from the European continent including Russia; six from the Middle East; and 18 from Asia.
"Spreading those votes among about 100 IOC members makes it very easy for a favorite - Rio or Chicago - to go out by one or two votes in that first round," said Ed Hula, editor, Aroundtherings.com.
It is the 16 IOC members from Africa who may carry crucial swing votes. President Obama has been making personal pitches to them -- no doubt hoping his own story will inspire them to back a U.S. city.
"The United States is eager to welcome the world to our shores," said Obama.
There is another factor that may influence the vote.
For the first time in decades, the International Olympic Committee has postponed bidding for the rights to broadcast the Olympics in the United States until after a host city is selected. Sports television executive Charlie Besser says that means a Chicago Games would mean a bigger payday for the IOC.
"The US rights are going to be in the $2 to 2.5 billion range. So a 20 percent premium is $400 to $500 million more that will be paid by the US broadcast company for a domestic games that would ordinarily be paid for the Games played in Rio, Madrid or Tokyo," said Charlie Besser, Intersport Founder & President.
So that's one way to handicap the race. But as an IOC member recently told ABC7, 'don't kid yourself into believing we always chose the best city or the one with the greatest plan for the Games.' Many more factors play a role - some pragmatic, others purely political.