(Adds details on Silva's requests, upcoming polls, Petrobras corruption case)
By Anthony Boadle
BRASILIA, Oct 9 (Reuters) - Environmentalist Marina Silva said on Thursday she will wait for political pledges from pro-business opposition candidate Aecio Neves before backing him in the Oct. 26 runoff to Brazil's presidential election.
An endorsement from Silva, who finished third in the election's first round on Sunday with 22 million votes, would increase Neves' chances of unseating leftist President Dilma Rousseff and ending 12 years of Workers' Party rule.
Silva's Brazilian Socialist Party backed Neves on Wednesday, but she wants to see him formally embrace some key points from her campaign platform before offering an endorsement.
In a note to parties that backed her unsuccessful presidential bid, Silva said her decision depended on Neves' response to her request.
A Silva aide, Pedro Ivo, said she is seeking "progressive" commitments to defend the rights of Brazilian Indians and landless peasants, issues that are not part of Neves' platform. She also wants Neves to commit to "sustainable development," a key concept of her campaign.
The Silva camp plans to deliver a list of requests to Neves' campaign managers on Friday, her spokesman Walter Feldman said.
"We hope Neves embraces the idea of political renewal," Feldman told reporters at a meeting at the Socialist Party's headquarters in Brasilia.
Silva, a popular anti-establishment figure, campaigned as an alternative to the two dominant political forces that have ruled Brazil for the last two decades, Rousseff's leftist Workers' Party and Neves' centrist Brazilian Social Democracy Party.
Silva also came in third in the last presidential election, when she ran on the Green Party ticket. In the 2010 runoff, she remained neutral instead of endorsing a candidate.
This time, though, Silva looks inclined to throw her weight behind Neves following an acrimonious campaign in which the Rousseff camp ran a flurry of attack ads portraying Silva as a serial flip-flopper beholden to a greedy financial elite.
Neves won 33.6 percent of the votes on Sunday to Rousseff's 41.6 percent, a difference of 8 million votes that he is trying to reduce by courting Silva's supporters and undecided voters.
The first polls of the second round are expected later on Thursday and will show whether Neves, an investor favorite who surged to second place in Sunday's vote, has continued to gain support and narrow Rousseff's lead.
The polls are likely to face greater scrutiny after failing to accurately gauge the extent of Neves' late surge in the first round, though they did flag the trend.
Neves, a senator and former state governor, is campaigning to reduce state intervention in the economy and restore investor confidence in Brazil, which slipped into recession last quarter under Rousseff.
The incumbent is vowing to expand the social programs that have lifted millions out of poverty and reduced inequality in Latin America's largest nation since the Workers' Party won office in late 2002.
Rousseff has a strong base of support among Brazil's lower classes but is also up against a widespread desire for change in a country where many Brazilians see their politicians as self-serving and corrupt.
A festering corruption scandal involving state-run oil company Petroleo Brasileiro SA and three political parties from the ruling coalition threatens to dog Rousseff for the rest of the campaign after testimony from a former Petrobras executive was leaked on Thursday.
In a recorded plea bargain that was aired by the GloboNews 24-hour news channel, Paulo Roberto Costa, the former head of Petrobras' refining and supply unit, said Rousseff's Workers' Party and two allied parties shared the proceeds of a 3 percent kickback from construction, shipbuilding and other contracts.
The Workers' Party and Petrobras have denied the allegations, dismissing them as a politicized smear campaign. The Neves camp has latched on to the case, using it to argue that Rousseff and the Workers' Party are soft on corruption.
"This is sad episode in the history of Petrobras and of Brazil. Corruption has always existed, but when it appears to be organized by party leaders it makes you angry," said Mansueto Almeida, a Neves campaign advisor.
"It's just an allegation and the courts will have to gather proof. But it's a serious problem for the current government." (Additional reporting by Maria Carolina Marcelo, Jeferson Ribeiro and Jeb Blount; Editing by Todd Benson and Cynthia Osterman)