COPENHAGEN- The four cities bidding for the prize honor of hosting the 2016 Summer Olympics began three intensive days of lobbying on Tuesday with the Obama factor looming large ahead of Friday's vote by the IOC.

The White House announcement on Monday that President Barack Obama would fly in to the Danish capital to appear before the International Olympic Committee in support of Chicago's bid for the Games was the news the other three candidates least wanted.

Madrid, Rio de Janeiro and Tokyo all fear that the IOC membership could be swayed by Obama's charismatic appeal, as they were four years ago when then British prime minister Tony Blair helped persuade them to choose London over clear favorites Paris for the 2012 Summer Games.

Two years later then Russian president Vladimir Putin was similarly persuasive when the Black Sea resort of Sochi, rank outsiders, won the right to stage the 2014 Winter Olympics.

Obama, who has close connections with the Windy City, had originally opted to stay at home to deal with his health care reform plans and announced that First Lady Michelle Obama would travel instead to Copenhagen.

Chicago bid leaders were delighted by Monday's news which means both Obamas will now be part of the team making the presentation to the IOC membership on Friday.

CLOSE CONTEST

Before the announcement of Obama's visit, the Olympic insider publication Around the Rings had rated Rio as marginal favorites over Chicago with Madrid and Tokyo close behind in a contest widely seen as the closest ever.

There was little doubt among observers that the Obama factor has tipped the odds in Chicago's favor but, in such a close contest, late lobbying has often been decisive and all four have to consider they still have a reasonable chance.

IOC president Jacques Rogge was due to arrive in Copenhagen later on Tuesday and most of the IOC's 115 membership are expected in the next two days.

Chicago, Rio and Tokyo all launched their last lobbying effort on Tuesday, mainly for the benefit of more than 1,000 accredited written media in the Danish capital and a host of major broadcasters.

Madrid, who have favored a more low-key approach, were due to start their lobbying in Denmark on Wednesday after Tuesday's planned arrival of King Juan Carlos and Spanish prime minister Jose Luis Zapatero.

Celebrities were also flooding in to support the various bids. Rio were wheeling out soccer great Pele on Tuesday in a street soccer photo opportunity while Chicago were parading former U.S. sprint world record holder Michael Johnson and Romanian gold-medal gymnast Nadia Comaneci.

(Editing by Tony Jimenez)