Brazilian opposition presidential challenger Marina Silva would defeat incumbent president Dilma Rousseff if a runoff vote were held between them, according to a poll taken by the Ibope opinion research institute. The poll showed Ms. Silva with support from 45% of respondents, versus 36% for Ms. Rousseff in a possible second round runoff vote.  In the first round vote scheduled for Oct. 5, Ms. Rousseff maintains a lead of 34% to 29% for Ms. Silva. Third place opposition challenger Aecio Neves got backing from 19% of respondents. According to Brazilian election rules, a runoff election is held if no candidate gets more than 50% of the vote in the first round. Ms. Silva, of the Brazilian Socialist Party, recently got the nomination from her party after the party’s initial candidate for the post, former Pernambuco governor Eduardo Campos, died earlier this month in a plane crash. The poll, sponsored by the Estado de S. Paulo newspaper and the Globo news organization, was taken among 2500 voters between Aug. 23-25 and has a margin of error of 2 percentage points. The poll was the first to register the level of support for Ms. Silva after her candidacy was officially registered with local elections authorities.