(Rewrites throughout to add poll details and context)
BRASILIA Aug 27 Environmentalist Marina Silva has surged to a strong second place in Brazil's presidential election and is set to defeat President Dilma Rousseff if the race goes as expected to a runoff, a new poll showed on Wednesday.
Silva would defeat Rousseff by 43.7 percent of the votes to 37.8 percent in a runoff, said the survey by polling firm MDA. It was the second poll in two days to show Silva ahead in such a scenario.
Both polls show a second round of voting is likely because Rousseff would not win 50 percent of the valid votes in the Oct. 5 election. The two top vote-getters would face off three weeks later.
Silva has clearly pushed the other main opposition candidate, centrist Aecio Neves, into third place and is luring away some of his supporters.
A lifelong defender of the Amazon rainforest and a popular figure among young voters, Silva has upended the race since becoming a candidate last week. Silva had been the running mate of the Brazilian Socialist Party's previous contender, Eduardo Campos, who was killed in a plane crash on Aug. 13.
Yet many political analysts say Silva has benefited from public sympathy over Campos' death, so her support could wane in coming weeks as emotions fade and her positions on issues become better known.
Both Rousseff and Neves have plenty of television ad time and more powerful and better-funded parties behind them to counter Silva's rise.
In the first round of voting, the poll showed Rousseff with 34.2 percent of votes, down from 36.2 percent in the previous MDA survey in early August. Silva had 28.2 percent of votes, and Neves notched 16 percent support, down from 22.1 percent in the last MDA poll.
The MDA poll commissioned by the transport industry lobby group CNT surveyed 2,002 people between Aug. 21 and Aug. 24 and had a margin of error of plus or minus 2.2 percentage points. (Reporting by Anthony Boadle; Editing by Grant McCool)