By Brian Winter
SAO PAULO (Reuters) - The Brazilian Social Democracy Party (PSDB), the country's biggest opposition group, would formally support environmentalist Marina Silva in a runoff vote for the presidency if its own candidate fails to qualify, a party source told Reuters.
Such an alliance would reduce President Dilma Rousseff's chances of winning a second term by bringing together large, disparate groups of voters who are clamoring for change after more than a decade of Workers' Party rule.
The election is being closely watched by investors who are also hoping for a change in government after almost four years of stagnant growth and state intervention in the economy under Rousseff's left-leaning administration.
"Brazil needs a change, a renewal. It cannot tolerate four more years" under Rousseff, the well-placed PSDB source said, speaking on condition of anonymity.
Silva was set to formally join the presidential race on Wednesday, accepting the nomination of the Brazilian Socialist Party (PSB) following the death of former state governor and party chief Eduardo Campos in a plane crash.
The accident threw the Oct. 5 election into disarray, causing some voters to switch candidates and also threatening to upset the carefully negotiated web of political alliances underpinning the campaign.
A recent poll put Rousseff comfortably in first place, with Silva and the PSDB's candidate, Senator Aecio Neves, running neck-and-neck for second place.
But Rousseff seems unlikely to win more than half of the votes so she would face the second-place finisher in a runoff on Oct. 26.
Under Campos' leadership, the PSB and the PSDB had similar centrist, pro-business platforms. They had forged some alliances in state and municipal races and were expected to back each other against Rousseff in an eventual runoff.
Silva, however, is likely to embrace a somewhat more leftist, anti-establishment agenda. She only backed the PSB last October in what she herself described as a temporary arrangement until she can formally found her own party.
Given Silva's reputation for unpredictable decisions, and the suspicion with which many in Brazil's traditional elite view her, some analysts had speculated the PSDB might stop short of a formal endorsement if she faces Rousseff in a runoff.
Not so, the party source said. "We expect Aecio to be in the runoff and then win the election. But if it's Marina, the PSDB will support her."
Polls have indicated that most Neves voters would support Silva in a runoff even without the PSDB's official endorsement. But its formal support would be significant because it would mobilize the PSDB's network of mayors, governors and legislators, which is significantly bigger and better-organized than the PSB's.
A poll released by Datafolha on Monday showed that in a runoff Silva would win the support of 47 percent of voters, compared to 43 percent for Rousseff.
The president leads Neves by a margin of 47 percent to 39 percent in a second round vote, Datafolha said.
(Editing by Todd Benson and Kieran Murray)