Brazil’s president Dilma Rousseff gained back terrain in a  national presidential election poll released Wednesday, but still trails Socialist Party challenger Marina Silva in a runoff vote scenario. The latest poll from the Ibope opinion research institute showed Ms. Rousseff, of the Workers’s Party, with support from 37%, versus 33% for Ms. Silva. Challenger Aécio Neves of the Social Democratic Party trailed in third place with 15%. In a runoff vote, however, Ms. Silva would defeat Ms. Rousseff by a margin of 46% to 39%. According to Brazilian election rules, a runoff election is held if no candidate gets more than 50% of the vote in the first round.  A previous poll by Ibope showed Ms. Rousseff with 34% in a first round vote, versus 29% for Ms. Silva and 19% for Mr. Neves. Ms. Silva, a former senator and well-known environmentalist, received the nomination from her party in August after the party’s initial candidate for the presidency, former Pernambuco governor Eduardo Campos, died in a plane crash.  Polls released late Tuesday in Brazil’s heavily-populated states of São Paulo and Rio de Janeiro showed that Ms. Silva leads the race in those regions. Brazil is scheduled to hold a first round presidential vote on Oct. 5.