By Anthony Boadle BRASILIA (Reuters) - Brazilian presidential hopeful Aecio Neves is heading into Sunday's election with a slight edge over environmentalist Marina Silva in the race to advance to an expected runoff against the incumbent, three new polls showed on Saturday.
But on the eve of the vote, it remained unclear who President Dilma Rousseff's adversary would be in a likely second-round vote later in the month, since the gap between Neves and Silva was within the polls' margin of error, setting the stage for Brazil's tightest election in decades.
Just a few months ago, many political analysts were predicting that Rousseff would cruise to re-election despite a weak economy and simmering discontent with the high cost of living and bad public services that triggered massive street protests a year ago.
The campaign was upended in August, when Silva was thrust into the race after her Brazilian Socialist Party's original candidate died in a plane crash. A popular anti-establishment figure, Silva quickly surged in the polls and at one point looked poised to end the leftist Workers' Party's 12-year grip on power in Latin America's largest country.
But an aggressive media blitz by the Rousseff campaign eroded Silva's popularity with questions about her ability to govern Brazil's unruly democracy without the backing of traditional parties. The Rousseff campaign also portrayed Silva as a serial flip-flopper backed by a greedy financial elite determined to undo popular social welfare programs.
Neves, a pro-business senator and former state governor from the centrist Brazilian Social Democracy Party, has risen steadily in the polls as Silva's support deflated. He performed well in the last television debate on Thursday, calmly hammering Rousseff over a multimillion-dollar corruption scandal at state-run oil company Petrobras.
Silva, who has looked frail in the home stretch of the campaign, shrugged off Saturday's polls.
"The Brazilian people have decided this election will go to a second round. We are sure we will be there," she told reporters in Sao Paulo.
If no candidate gets more than 50 percent of valid votes on Sunday, the race will be decided in a runoff on Oct. 26 between the top two vote-getters.
ROUSSEFF FAVORED IN RUNOFF
Whoever clinches second place on Sunday faces an uphill battle to unseat Rousseff in a runoff. Saturday's polls, by the Datafolha, Ibope and MDA polling firms, showed Rousseff with a 6 to 10 percentage-point advantage over Neves in a second round. The polls also showed Rousseff beating Silva in a runoff.
First-round support for Rousseff held steady at 40 percent in all three polls, while Neves gained between 3 and 5 percentage points and Silva dropped between 2 and 4 percentage points, bumping her into third place for the first time.
If spoiled and blank ballots are excluded, Ibope showed Rousseff getting 46 percent of valid votes in Sunday's first round, 4 percentage points short of outright victory, and Neves with 27 percent and Silva with 24 percent.
MDA, which surveyed 2,002 voters on Thursday and Friday, projects that the election is most likely to be decided in a runoff between Rousseff and Neves because his rejection figures have declined markedly while Silva's negatives have risen.
Brazil's electoral authority said it might take longer than usual to call Sunday's vote, given the tight race for runner-up.
"This is an exciting election and the first since 1989 where we see the polling firms unable to make a solid prediction," Jose Antonio Dias Toffoli, president of Brazil's electoral court, told Reuters.
The MDA poll has a margin of error of plus or minus 2.2 percentage points.
Ibope polled 3,010 voters between Thursday and Saturday and Datafolha surveyed 18,116 respondents on Friday and Saturday. Both polls have a margin of error of plus or minus 2 percentage points.
(Additional reporting by Jeferson Ribeiro and Bruno Federowski; Editing by Todd Benson and James Dalgleish)