Brazilian politician Marina Silva (centre) attends the wake for late presidential candidate Eduardo Campos, at the Pernambuco Government Palace in Recife. Picture: Ricardo Moraes

Brasilia, Brazil -

Environmentalist Marina Silva could unseat incumbent Dilma Rousseff in Brazil's October presidential vote, a poll found on Monday, a stunning political twist after Silva's running mate was killed in a plane crash.

Silva, a 56-year-old former environment minister, has moved into second place ahead of the October 5 first-round vote and would defeat Rousseff in an October 26 runoff, the survey by Datafolha found.

It was the first poll taken after Socialist Party candidate Eduardo Campos's death in the August 13 plane crash.

Silva was Campos's vice-presidential running mate, and is widely expected to be chosen to replace him at the top of the ticket when party leaders announce their decision on Wednesday in Brasilia.

The campaign officially kicks off on Tuesday, when candidates begin taking to the airwaves to pitch their platforms.

The poll found Silva had 21 percent support against 36 percent for Rousseff and 20 percent for Social Democrat Aecio Neves.

In a runoff vote, Silva would defeat Rousseff 47 percent to 43 percent, it found.

The incumbent, however, would beat Neves 47 percent to 39 percent if they faced each other in the second round.

The Datafolha poll was carried out August 14 and 15 - the two days following Campos's death - and has a margin of error of two percentage points.

In a rare glimmer of good news for Rousseff, it also found the president's approval rating had risen from 32 percent to 38 percent.

Campos had been running in third place before his campaign jet crashed en route to Sao Paulo, killing all seven people on board.

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

Silva had been considered a top contender for this year's race after surprising many pundits by coming in third in 2010 with 19 percent of the vote running on the tiny Green Party's ticket.

But Brazil's electoral court ruled last October that she had failed to collect enough signatures to register her new party, Sustainability Network, in time.

She then opted to join forces with the affable, politically connected Campos and his PSB party.

With her compelling personal story and broad-based appeal, she could be even more effective than her late running mate at selling the PSB's message of change after 20 years of government by Rousseff's Workers' Party (PT) and Neves's PSDB.

Born into a poor family of rubber tappers in the Amazon, she only learned to read and write at 16 years old, the start of a meteoric rise to become a figurehead of the country's environmental movement.

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

Monday's poll found Silva had broad support from undecided voters and those who had previously planned to cast “blank ballots” under Brazil's compulsory voting system.

“That's a sign that voters perceive her as an alternative,” said analyst Rafael Cortez of consultancy Tendencias.

But others said the poll numbers could be fleeting.

“It's still too early to know whether these figures are firm and will continue once the emotional moment of Eduardo Campos's death has passed,” said Andre Perfeito, chief economist at Gradual Investimentos.

The poll nevertheless had an impact on the Sao Paulo stock market, where shares rose 1.05 percent on Monday.

Brazil's economy, which grew rapidly under Rousseff's popular predecessor and mentor, Luiz Inacio Lula da Silva, hitting 7.5-percent growth in 2010 - has been sluggish under Rousseff, registering an average 2.1-percent growth from 2011 to 2013.

Monday's poll threw uncertainty on the election outlook but also boosted hopes of a leadership change, said economist Silvio Campos Neto of Tendencias.

“The market took this signal calmly, because it views any situation that allows a change in government as positive,” he told AFP.

“Markets prefer Aecio, but Marina showed she is well positioned and has a chance. She's seen as a competent candidate.” - Sapa-AFP