By Brian Winter
SAO PAULO (Reuters) - Brazil's biggest opposition party would support Marina Silva for president if its own candidate fails to make an October runoff, a party source told Reuters, an alliance that would damage President Dilma Rousseff's chances for a second term.
The presidential race was upended last week by the late entry of Silva, a popular environmentalist and anti-establishment icon among many young voters, following the death of her party's previous candidate in a plane crash.
A recent poll showed Rousseff in first place for the Oct. 5 vote, with Silva and Senator Aecio Neves of the centrist Brazilian Social Democracy Party (PSDB) in a statistical tie for second. The top two finishers would face each other in an Oct. 26 runoff if no one receives more than half of valid votes.
The PSDB is fully confident that Neves will make the runoff, party leaders said on Wednesday.
But if he doesn't, the party would throw its considerable organizational structure and popular support behind Silva in an effort to oust Rousseff and revive Brazil's economy, a well-placed party official said.
"Brazil needs a change, a renewal. It cannot tolerate four more years" under Rousseff, the source said, speaking on condition of anonymity.
Many investors are tired of slow growth and high inflation under Rousseff's left-leaning policies. Local stocks have jumped 6 percent in the last week on increasing hopes of a victory for Silva or Neves, who both say they would intervene less in the economy and be more friendly to business.
In a statement e-mailed to Reuters, the PSDB said the report of support for Silva was "not accurate," adding that the party "has never considered the possibility" of Neves not winning the election. It did not provide further details.
The party's support for Silva in an eventual runoff against Rousseff had previously been a question mark.
Many within Brazil's political establishment distrust Silva's past record of unpredictable decisions as well as her emphasis on environmental sustainability, even at the cost of economic growth. She would likely govern significantly to the left of the Brazilian Socialist Party (PSB) presidential candidate who she abruptly replaced, the late Eduardo Campos.
Yet, by drawing clear battle lines now and signaling it would support Silva, some in the PSDB may be hoping to deflect attention from the race for second place and more firmly cast the election as a referendum on Rousseff's record.
Neves, 54, told reporters on Wednesday that Silva's entry into the campaign "doesn't change absolutely anything."
"Our adversary is the government," he said.
SILVA MAY SURGE FURTHER
Neves is a former governor of Minas Gerais state, one of Brazil's largest, and casts himself as the best administrator in the race. He has promised to undertake the pro-market reforms he says are necessary to revive Latin America's largest economy.
PSDB officials have privately said they expect Silva to continue to gain support in upcoming polls following a public outpouring of emotion over the death of Campos, a former governor of Pernambuco state who was widely seen as one of Brazil's brightest young politicians.
However, they also say that support for Silva may then fade as voters learn more about her platform and as Neves touts his record.
Neves has significantly more allotted time than Silva for free television ads, which have historically been the main way Brazilian voters learn about their candidates. The PSDB also has a broader, more nationwide network of mayors and governors.
Silva was expected to officially accept the PSB's nomination for president later on Wednesday.
In the first major poll since Campos' death, Datafolha pollsters said on Monday that Rousseff would receive 36 percent of the vote in the first round. Silva and Neves had 21 percent and 20 percent, respectively.
But in a runoff, Silva would take 47 percent of votes 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.
Neves' running mate, Aloysio Nunes, said on Wednesday that their ticket "has the conditions" to perform even better in the first round than the PSDB's candidate in 2010, when former Governor Jose Serra won 33 percent of the vote. Serra then lost to Rousseff in a runoff.
"Aecio has the administrative capability to be an excellent president," Nunes said in a brief telephone interview. "Talking of other scenarios isn't worth much at this time."
(Editing by Todd Benson and Kieran Murray)