When Marta Suplicy, a Michigan State-educated sexologist, was elected mayor of South America's largest city, the talk was that her next stop would be Brazil's presidency.

Four years later, she has felled Sao Paulo's urban bus mafias, created a record number of preschools for poor children and, some believe, been Paulistas' best mayor ever.

Yet Suplicy, 59, a former TV commentator on sexual behavior, has also grown a reputation for arrogance and aloofness. Moreover, she's done exactly what Brazilian wives aren't supposed to do: shed her popular spouse, a Brazilian lawmaker, for a handsome Franco-Argentine husband.

The net result: Polls place her well behind a plodding challenger in a runoff election Sunday. Her defeat would be a blow to the ascendant Workers' Party, or PT, and the leftist president, Luiz Inacio Lula da Silva, who would no longer be able to count on carrying Brazil's financial center, home to 12 million people -- 18 million counting peripheral cities -- when he seeks reelection in 2006.

But in nearby Uruguay, the leftist presidential front-runner, Tabare Vazquez, appears headed for an outright victory Sunday.

Vazquez, a radiologist, closed his campaign promising to place relations with South American neighbors ahead of ties with Washington and assuring nervous business leaders that change in Uruguay, if he won, would be progressive rather than radical.

If Vazquez fails to win a majority of the vote on Sunday, there will be a runoff in November.

A Vazquez election would likely cost Washington another ally in South America. His Frente Amplio, or Broad Front, is a coalition of leftist forces that includes moderates, radicals and even guerrilla leaders.

Vazquez patterns himself after Brazil's Lula, promising an open economy but with a new covenant to help Uruguay's poor. "It's not enough to have economic growth. You have to distribute wealth throughout all of society," he has said.

The United States has long counted on the vote of conservative Uruguayan governments in the United Nations to denounce Cuba's human rights record. Uruguay sponsored resolutions in 2002 and 2003 condemning the lack of liberties in Cuba. Working closely with the United States, Uruguay, a country of 3.4 million, last year provided more than 1,700 peacekeepers to 11 U.N. missions.

Asked last Wednesday about relations with the United States, Vazquez answered the question by not answering it. During a lunch with business leaders, he said his top foreign policy priority was political and economic integration with Uruguay's neighbors. They are Argentina, Brazil and Paraguay, with whom Uruguay formed the Southern Cone Common Market, or Mercosur, which took effect in 1994.

Uruguay's outgoing president, Jorge Batlle, has been a staunch U.S. ally and, as such, an odd man out at regional gatherings. On Monday, the Bush administration and Batlle's government signed a bilateral treaty safeguarding investment and promoting exports. Trade between the United States and Uruguay last year topped $582 million.

Vazquez, a vocal critic of the U.S.-led invasion of Iraq, has lost two presidential bids. But Batlle's Colorado Party is reeling after an 11 percent economic slide in 2002. Jorge Larranaga, the candidate of the other traditional party -- the White, or National, Party -- trails by at least 20 percentage points in opinion polls.

In Sao Paulo last week, Suplicy broke down and cried for 15 minutes during an appearance before senior citizens, saying that she was a victim of prejudice as a female mayor and had received unfair news coverage.

"What I have felt in prejudice, in persecution by the press, is very difficult to accept," she told the stunned seniors. "What I have been accepting is really very difficult."

The front-runner, Jose Serra, said in an interview that Suplicy was not being discriminated against as Sao Paulo's second female mayor.

"This race is about administrative problems and our [different] styles of working. That's it," Serra said.

The stakes are high because as Sao Paulo goes, politically speaking, so goes the nation, said Joao Paulo Lima e Silva, who was recently reelected mayor of Recife, a sprawling city in northern Brazil and the capital of the state of Pernambuco.

"In a way," he said, "Sao Paulo determines the politics of Brazil, because of the number of boroughs and because of its economic power. So for us in the Workers' Party and leftist forces it is fundamental to reelect Marta."

Lula crushed Serra in the 2002 presidential race, but the bland former federal health secretary appears set to take revenge in Sao Paulo.

"I think the most probable scenario is a loss for the PT. Serra is showing a consistent advantage and it should hold," said Alan Lacerda, a political analyst with Goes & Consultores in Brasilia, the capital.

The charge of arrogance stuck to Suplicy after she shouted down a poor woman who had complained about her government.

Suplicy took another hit when she left her longtime husband, Sen. Eduardo Suplicy, and married Luis Favre in September 2003.

Male politicians in Brazil routinely marry younger women, but many Paulistas sided with Suplicy's husband and frowned on her lavish wedding, attended by South America's elite.

"I think there is a lot of machismo," complained Ana Paula Nascimento Santos, a college student who supports Suplicy's campaign. "There's a lot of prejudice."

For Suplicy, whose slogan is "A Woman of Courage," the toughest moment may have been when her publicity chief, Duda Mendonca, was arrested this month at an illegal cockfight in the state of Rio de Janeiro after allegedly trying to use his presidential connections to avoid arrest. Images of his arrest were broadcast widely. He was fired the next day.

Suplicy's opponent, Jose Serra, flashed a victory sign after the first-round vote Oct. 3. Serra leads in pre-runoff polls.Sao Paulo Mayor Marta Suplicy, greeting crowds, trails in polls ahead of a runoff election Sunday, which is being seen as a bellwether for Brazilian politics. Jorge Larranaga, of Uruguay's White Party, lags significantly going into Sunday's presidential election.A victory by Tabare Vazquez, of the Broad Front coalition, could cost Washington a longtime regional ally.