(Add Neves criticism of Silva, paragraph 5)
By Anthony Boadle
BRASILIA, Aug 27 (Reuters) - Environmentalist Marina Silva would handily beat President Dilma Rousseff if Brazil's October election goes to a runoff, a poll showed on Wednesday, an outcome that seemed unimaginable just a few weeks ago and one that would put an end to 12 years of Workers' Party rule.
Silva would win 43.7 percent of the votes to 37.8 percent for Rousseff in a runoff, said the survey by polling firm MDA. It was the second poll in less than 24 hours to show Silva ahead in such a scenario.
Both polls point to a probable runoff because Rousseff looks unlikely to win more than 50 percent of 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 and market favorite, centrist Aecio Neves, into third place and is luring away some of his potential voters, the polls showed.
To keep alive his chances, Neves has had to target Silva. In an interview webcast by O Estado de S.Paulo newspaper he focused on her lack of executive experience. Neves said he was better prepared to run the country and added: "Brazil is not for amateurs."
After four years of sluggish economic growth under Rousseff, many investors are hoping the election will bring in a new president who will put Brazil on a more market-friendly track, helping to lure investments needed to revive the world's seventh-largest economy.
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 major policy positions become better known.
Both Rousseff and Neves have more allotted campaign time on television and better-funded parties behind them to counter Silva's rise.
The MDA poll showed Rousseff would garner 34.2 percent of votes in the Oct. 5 election, down from 36.2 percent in the previous MDA survey in early August. Silva had 28.2 percent, and Neves 16 percent, down from 22.1 percent in the last MDA poll.
An Ibope poll published on Tuesday showed Silva defeating Rousseff by a nine-point margin in a runoff.
Silva appeals to voters who are disenchanted with Brazil's political establishment and see her as a principled outsider who can bring ethics to government. Both polls showed her attracting disenfranchised and uncommitted voters.
An evangelical Christian, Silva can also tap the votes of this growing religious constituency. The MDA poll showed support for another evangelical candidate, Pastor Everaldo, slumped to 1 percent from 3 percent.
The prospect of a Silva victory has rallied Brazil's stock market for two weeks, exciting investors who are frustrated with what they see as Rousseff's interventionist economic policies.
Sao Paulo's main stock index rose 2 percent by midafternoon, with scandal-hit state-run oil firm Petroleo Brasileiro SA up by more than 4 percent.
"Investors are expecting changes in the running of the company if a new government takes office," HSBC analyst said in a note to clients.
Yet Silva's intransigent style of politics may make it difficult for her to build the sometimes murky coalitions that presidents need to govern Latin America's largest nation.
"Marina has a lot of challenges ahead. Her party is divided and she has conflicts with political alliances in big electoral districts such as Sao Paulo," said Andre Cesar, a Brasilia-based political consultant.
Cesar said that if Silva were elected, she would have to cozy up to Brazil's largest party, the PMDB, which controls both chambers of Congress. "Who will she govern with? She will have to sit down with these guys and negotiate. Will she be able to stomach that?"
In the first presidential debate of the campaign on Tuesday night, rival candidates sought to highlight Silva's past run-ins with Brazil's powerful agribusiness sector, which accounts for almost one quarter of the country's economic output.
A confident Silva brushed off the criticism and said economic growth and conservation are not incompatible.
While Silva's economic policies remain unclear, her adviser, Eduardo Giannetti, has said they will be as orthodox and market-friendly as those of Neves, the candidate for the Brazilian Social Democracy Party.
The MDA poll, commissioned by the transport industry lobby group CNT, surveyed 2,002 people between Aug. 21 and Aug. 24. It had a margin of error of plus or minus 2.2 percentage points. (Additional reporting by Asher Levine and Aluisio Alves in Sao Paulo; Editing by Grant McCool)