Popular environmentalist Marina Silva has shaken up Brazil's presidential election since her sudden entry into the race, surging to a surprise lead in opinion polls and impressing in her first debate.

Silva, who says she wants to be Brazil's first "poor, black" president, has emerged as a serious threat since taking over the top spot on the Socialist ticket following her late running mate Eduardo Campos's death in a plane crash on August 13.

Several polls since Campos's death have found Silva, a 56-year-old former environment minister, would unseat incumbent Dilma Rousseff in an October 26 runoff election.

The latest, released Wednesday by polling firm MDA, found Silva would beat Rousseff 43.7 percent to 37.8 percent.

Silva also delivered a commanding performance in the first presidential debate Tuesday night, forcefully criticizing Rousseff and Social Democrat Aecio Neves, the candidate who had been in second place until Campos's death.

Rousseff, who has presided over sluggish economic growth and rising inflation, is a "manager with no strategic vision" who has committed "clear mistakes" in her handling of the economy, Silva said.

Dressed in a crisp white suit, she avoided policy specifics, but promised a "new politics" after 20 years of government by Rousseff's Workers' Party (PT) and Neves's PSDB.

"Marina was good in the debate. She surprised. She showed a sureness she didn't have before, mature," said political analyst Andre Cesar of consultancy Prospectiva.

- Stunning rise -

Silva's compelling personal story makes her an appealing candidate for a broad swathe of voters.

Born into a family of rubber tappers deep in the Amazon, she grew up in poverty, helping collect rubber from an early age and later working as a maid.

She only learned to read and write at 16, when she fell ill with hepatitis and went for treatment to Rio Branco, the capital of Acre state, where she enrolled in a literacy program.

She also took a class on rural union organizing with Chico Mendes, the famed environmentalist who was assassinated in 1988 for his work defending the Amazon.

Silva joined his movement and rose to be a leader in her own right during a campaign of peaceful resistance to deforestation.

In 1994 she was elected Brazil's youngest-ever senator at the age of 36, running on the PT ticket.

She won re-election in 2002, then was picked by Rousseff's popular predecessor and mentor, Luiz Inacio Lula da Silva, to be his environment minister in 2003.

She succeeded in curbing deforestation but left the PT in 2009, saying the party was too focused on economic growth at the expense of the environment.

The following year she ran for president on the tiny Green Party's ticket, surprising many by coming in third place with 19 percent of the vote.

Rousseff, now 66, won the election to become Brazil's first woman president.

- Turbulent times -

Silva had planned to launch her own party, Sustainability Network, for this year's race.

But Brazil's electoral court ruled last October that she had failed to collect enough signatures to register it in time.

She then opted to join forces with the affable, politically connected Campos and his Socialist Party (PSB) -- which named her its candidate after his death.

An evangelical Christian, Silva appeals to both religious conservatives and the left.

Polls have found she is also capturing a large number of undecided and alienated voters.

The election comes at a turbulent time for Brazil, which enjoyed an economic boom under Lula but was shaken last year by social unrest that exploded into massive protests.

Hundreds of thousands of people hit the streets in June 2013 to protest the cost of hosting the World Cup and the lack of investment in education, health and transport.

Silva has largely managed to stay above the fray.

She has also injected emotional energy into what had been a stagnant race, with Rousseff at the time on track for re-election, despite widespread discontent.

"Marina has a good chance of winning," said analyst Andre Perfeito of consultancy Gradual Investimentos.

"She's a woman, she's black, and she has an interesting story after Campos's death."