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Environmental icon Marina Silva may struggle to advance her environment agenda if elected president. Leo Correa/AP

Marina Silva rose from poverty deep in the Amazon jungle to become a prominent politician and the advocate who kick-started Brazil’s battle against deforestation. Now she is clashing with an old foe, President Dilma Rousseff, in a wild election that could reshape the nation’s environmental policy.

Silva’s sudden ascent is almost as shocking as the event that prompted it: a plane crash on 13 August that killed presidential candidate Eduardo Campos. That pushed Silva, his running mate, to the top of the centre-left Brazilian Socialist Party. Although the Campos-Silva ticket had struggled to gain traction, Silva is now in a statistical dead heat with Rousseff going into the election on 5 October; the two women are expected to face off in a runoff vote three weeks later.

As a third-party contender, Silva has promised a break from traditional politics amid growing anger over Brazil’s stagnant economy and political corruption that has tainted the Workers’ Party led by former president Luiz Inácio ‘Lula’ da Silva and his successor, Rousseff. But the campaign has also shown that Silva’s grassroots ‘socio-environmentalism’ has wider appeal than many once believed.

Almost one-fifth of Brazilians supported Silva when she ran for president under the Green Party banner in 2010, but she seemed destined to remain a protest vote. After briefly trying to form her own party, she joined forces this year with Campos, a popular governor from the northeastern state of Pernambuco and Lula’s former science minister.

“There has been a sea change in the political culture in Brazil, and Marina is one of the people who has brought it about,” says Steve Schwartzman, an anthropologist with the Environmental Defense Fund, a non-profit advocacy group in New York, who began working with Silva in the 1980s.