Istanbul, Madrid and Tokyo will make 45-minute presentations Saturday morning ahead of the vote later in the day by the International Olympic Committee.
Leading the delegations will be the prime ministers of all three countries. Shinzo Abe of Japan, Mariano Rajoy of Spain and Recep Tayyip Erdogan of Turkey all flew to Buenos Aires straight from the G20 summit in St. Petersburg, Russia.
"It's certainly an open race," South African IOC member Sam Ramsamy said Friday. "They all have positive and negative points. The final presentations will be crucial."
Picking the city with the least risks shapes up as the challenge for the IOC.
Tokyo has been seen as a slight favorite, but its status has been put into question by concerns over the leak of radioactive water from the crippled Fukushima plant.
Madrid, once counted out because of Spain's financial troubles, has generated the most recent buzz and momentum and could be poised for an upset win.
Istanbul, dogged by the war in neighboring Syria and possible Western military strikes against Bashar Assad's regime, looks like the outsider.
London bookmakers have been taking a rush of bets on Madrid, whose odds have been slashed from 4-1 a week ago to 5-4. Tokyo remains the betting favorite, though its odds have shortened to 5-6. Istanbul is listed at 6-1.
"There are all kinds of predictions, all kinds of bets but the only one numbers that matter are the ones from the votes on Saturday afternoon," Madrid Mayor Ana Botella said.
IOC elections are extremely unpredictable as members vote by secret ballot and take different personal reasons into account. Some members are still undecided and will be waiting for the final 45-minute presentations before making up their mind.
If all IOC members are in attendance, 97 will be eligible to vote in the first round. With a majority required for victory, the process is likely to go two rounds. The city with the fewest votes is eliminated after the first round, setting up a final head-to-head ballot. Outgoing IOC President Jacques Rogge will open a sealed envelope to announce the winner.
All three are repeat bidders: Istanbul for a fifth time overall, Madrid for a third straight and Tokyo a second in a row.
Previous bid campaigns have been marked by overriding geographic or emotional factors. In 2009, the IOC awarded the 2016 Olympics to Rio de Janeiro because of the Brazilian city's push to take the games to South America for the first time.
There has been no feel-good theme this time as the campaign has been dominated by the negatives surrounding each bid: Syria, doping scandals and anti-government protests in Turkey; severe recession and 27 percent unemployment in Spain; and, most recently, the Fukushima leak in Japan.
"Some of the issues are big and difficult for IOC members: geopolitical, environmental and economic," Australian IOC member John Coates said.
Many members will also be voting with Rio in mind. The Brazilian city has been plagued by construction delays and other issues in its preparations for 2016, leaving members in search of a safe, reliable host for 2020.
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"With the difficulties in Rio, many people will be thinking, 'Do we need to be with safe hands or take a few risks?" Coates said.
Tokyo has championed itself as the "safe pair of hands" but has been on the defensive this week because of Fukushima. Some IOC members are looking for Abe to deal with the issue directly in Saturday's presentation.
On Friday, Tokyo organizers gave the clearest response so far of the leak.
Hiroshi Hase, a former Olympic wrestler and member of the Japanese parliament, minimized the problem with a detailed presentation
-- a strategy that has been lacking for several days as Japanese officials struggled with the subject.
"There is a very limited area where contaminated water is leaking out in the port. ... But we will monitor it, remove it and contain it. That is the gist of the strategy," Hase said.
Madrid has gained the most ground in recent months, weeks and days. The turnaround started at the bid city technical presentations in Lausanne, Switzerland, in July, where the Spaniards pressed their case that they offer the safest financial option: 80 percent of the venues ready and only $1.9 billion needed for construction.
"There was a buzz about Madrid in Lausanne and I think there may be some continuation from that," Coates said.
Madrid also has a star performer in Crown Prince Felipe, a former Olympic sailor and Spain's flag-bearer at the 1992 Barcelona Games. He wowed the members in his speech in July and will be Madrid's featured speaker in Saturday's presentation.
"He's an Olympian, which is always a great advantage," Coates said. "People have an empathy with someone who is one of theirs."
Istanbul is pitching its case as an "historic choice" for the IOC: the first Olympics in the region, the first in a city linking two continents, the first in a predominantly Muslim nation.
"Istanbul 2020 will be held against the backdrop of one of the most magical cities on the planet: A bridge between continents, cultures and generations for thousands of years," bid leader Hasan Arat said.
French IOC member Jean-Claude Killy said delegates may need to put all the current issues aside and look ahead to the situation seven years from now.
"You have to project yourself to 2020," Killy said. "There's still seven years to get there. What's going to happen in those seven years in those three cities and those three countries? Who deserves it more, putting aside the problems?"
Press; By STEPHEN WILSON]
AP Sports Writers Tales
Azzoni and Stephen Wade contributed to this report.
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