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Eugenio Savio, Associated Press
Aecio Neves, presidential candidate of the Brazilian Social Democracy Party, PSDB, flashes victory signs after voting in general elections in Belo Horizonte, Brazil, Sunday, Oct. 5, 2014. An exit poll showed Sunday that President Dilma Rousseff will face challenger Aecio Neves in a second-round vote in Brazil's most unpredictable presidential election since the nation's return to democracy nearly three decades ago.

RIO DE JANEIRO — An exit poll showed Sunday that President Dilma Rousseff will face challenger Aecio Neves in a second-round vote in Brazil's most unpredictable presidential election since the nation's return to democracy nearly three decades ago.

The poll by the respected Ibope Institute found that Rousseff won 44 percent of the vote against Neves' 30 percent. The poll interviewed 64,200 people across Brazil. It had a margin of error of 2 percentage points.

The exit poll indicates a stunning fall for another challenger, former environment minister Marina Silva, who in late August held a commanding lead in polling but took just 22 percent in the exit poll.

Rousseff's aggressive campaigning eviscerated the support of Silva, who only entered the race in mid-August following a plane crash that killed her Socialist Party's original candidate.

It was thought Silva would tap into the widespread disdain Brazilians hold for the political class — anger that boiled over into roiling, nationwide anti-government protests last year. Opinion polls taken just after the demonstrations over a year ago indicated Silva was among the few political figures unscathed, given her squeaky clean reputation amid what voters say is a sea of corruption.

But Silva has not withstood a barrage of attacks labeling her as indecisive and without the mettle needed to lead the globe's fifth-largest nation — the message pounded on by Rousseff and the other top opposition candidate, Aecio Neves of the Social Democracy Party.

"Marina Silva tried but was not able to convey her message of change. She's only responding to attacks," said Paulo Sotero, director of the Brazil Institute at the Woodrow Wilson International Center for Scholars in Washington. "We've seen negative campaigning before, but never at this level of ferocity."

Rousseff's support rose to 46 percent in a survey released hours before the vote. But even the leader said it was unlikely she could push through to win the absolute majority required to avoid a second-round election on Oct. 26.

"I'm not operating with that idea; I'm working with the idea there will be a runoff," Rousseff said just before casting her vote in southern Brazil, where she lived for many years and first entered politics.

The race for second place between Silva and Neves, the former governor of Brazil's second-biggest state who comes from a storied political lineage, was deadlocked, though momentum clearly swung in Neves' favor in the past week as he gained support in surveys.

"The fear campaign that Dilma and her marketing people have set up against Marina Silva has had a strong effect," said David Fleischer, a political science professor at the University of Brasilia. "Dilma's people are saying Marina will abolish ... things they've gained through government social programs."

That's the heart of an apparent contradiction in the unpredictable campaign.

Opinion surveys indicate around 70 percent of Brazilians say they want change — as made plain by the mammoth anti-government protests last year blasting Brazil's woeful public services despite the nation's heavy tax burden.

Yet surveys also say nearly three-fourths of Brazilians express satisfaction with their lives.

"They want more of the same, and that is what Dilma is offering," Fleischer said.

During nearly 12 years in power, the Workers' Party has ushered in strong social programs that have helped lift millions out of poverty and into the middle class. Rousseff's strongest support comes from the poorest, those who are precariously hanging onto gains amid an economy that has sputtered the past four years.

"I don't think a sudden change would be good for the country. That could be dangerous," said Diego Almeida, a 26-year-old university student and resident of Rio's biggest slum who said he voted for Rousseff.

Still, he expressed the frustration millions of Brazilians have with their leaders: "They've had 500 years to fix this country and for 500 years they've failed. I just hope that something happens in the next 500 years."

Rousseff promised to expand social programs and continue strong state involvement in the economy, even though critics complain it creates a poor business environment and the main stock market tumbled every time a new poll showed her on the rise.

Both Silva and Neves offered more centrist economic approaches, such as central bank independence, more privatizations and the pursuit of trade deals with Europe and the United States.

After voting, Silva said she was "confident we will get to the second round. We put up a good fight and, God willing, we'll reach the runoff."

Neves said he was optimistic that he would be Rousseff's opponent in the second round. He said he'd "love to talk" to Silva about gaining her support.

Associated Press writers Jenny Barchfield in Rio de Janeiro and Stan Lehman in Sao Paulo contributed to this report.

Brad Brooks on Twitter: www.twitter.com/bradleybrooks