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 PSDB is fully confident that Neves will make the runoff, party leaders said on Wednesday.