SAO PAULO - Brazilian President Dilma Rousseff is set for a bruising defeat in an October runoff election against popular environmentalist Marina Silva, a poll suggested Tuesday.

The survey by polling firm Ibope found Rousseff would come out ahead in the first-round election on October 5 but be forced to face Silva in a second-round vote three weeks later, losing 36 percent to 45 percent. Brazil’s political landscape has been deeply shaken by the death of Socialist presidential candidate Eduardo Campos in a plane crash on August 13, unlocking an unexpected and vigorous challenge by Silva, his former vice presidential running mate. Silva, a 56-year-old former environment minister, is set to take 29 percent in the first round, against 34 percent for Rousseff, according to the poll. Social Democrat Aecio Neves would finish third with 19 percent. Going head-to-head in the runoff, Silva would unseat Rousseff, the poll found. It was carried out between August 23 and 25 and has a margin of error of two percentage points.

Campos’s death has dealt a surprising blow to the incumbent, who is seeking a second four-year term, and her Workers’ Party (PT), in power since Rousseff’s popular predecessor and mentor Luiz Inacio Lula da Silva took office in 2003.

Before Campos died, the election outlook had been stable for months, with Rousseff polling about 36 percent, Neves 20 percent and Campos eight percent.

The first poll after the plane crash, a Datafolha survey published on August 18, found Silva would beat Rousseff in a runoff by a score of 47 percent to 43 percent.

Silva has broad appeal in Brazil with her environmentalist credentials, evangelical Christian beliefs and compelling personal story.

Born into a poor family of rubber tappers deep in the Amazon, she only learned to read and write at 16 years old.

But she rose to become a leading figure in the country’s environmental movement and came third in the 2010 presidential election with 19 percent of the vote running on the tiny Green Party’s ticket.