SAO PAULO, Sept 1 (Reuters) - Popular environmentalist Marina Silva looks capable of winning Brazil's presidential election in October but a major campaign gaffe and mounting attacks from other candidates and the media suggest the race is still wide open.
Polls have shown Silva with a lead of about 10 percentage points over President Dilma Rousseff if the Oct. 5 election goes to a runoff, as seems likely.
Silva's meteoric rise has led Brazilian stocks to rally 10 percent in the last three weeks on hopes she would be more business-friendly than Rousseff and help stir a stagnant economy.
In the last week, Silva has successfully begun to address some of the doubts voters have about her - namely, whether she has the personal gravitas and organizational support to govern this continent-sized nation of 200 million people.
An anti-establishment icon who grew up poor in a family of rubber tappers, taught herself to read as a teenager, and then became a leading advocate for the Amazon rainforest, Silva has inspired many Brazilians with her earnest demeanor and clear voice on moral issues.
Yet many voters have been unconvinced that a great life story would make for a great president.
Silva, 56, has struggled during her political career to recruit and maintain allies, having quit two parties since 2009 and failed to organize a third.
Her unpredictable record of decision-making, shifting views and even her frail physical figure - the product of malaria and mercury poisoning during her youth - have also raised red flags.
To that end, Silva has struck a more sober and inclusive tone in TV ads and on social media compared with her sometimes amateurish presidential campaign in 2010, when she focused overwhelmingly on "green" issues and placed third.
She has also vowed to restore order to deteriorating public finances, and met with a wide variety of investors - including a dinner on Friday with leaders from the agribusiness sector, which accounts for about a quarter of the economy and has historically opposed her with great vigor.
"A lot of myths were broken," Luiz Cornacchioni, executive director of the Brazilian Agribusiness Association, told Reuters after attending the dinner.
"She showed herself to be a person who is open to dialogue and who knows about the importance of agribusiness," Cornacchioni said. "I left with a very positive view."
Perhaps the clearest sign of Silva's growing heft came in last week's presidential debate, when she directly asked Rousseff why she didn't follow through on reforms she promised after huge street protests swept Brazil in 2013.
Rousseff replied that Congress had blocked some proposals, but other public grievances like poor schools have improved. Silva calmly turned to the camera and said: "It's important for us as politicians to recognize our problems ... This colorful Brazil that President Dilma described exists only in the movies."
SUDDEN REVISIONS TO PLATFORM
Silva's greater maturity suggests her rise in polls has staying power, and is more than the result of public sympathy following the death of her Brazilian Socialist Party's previous candidate, Eduardo Campos, in a plane crash last month.
Yet the growing view on Wall Street and in Sao Paulo's financial circles that Silva already has the election wrapped up seems premature.
The last week has seen evidence of why some observers think Silva's support could still decline almost as fast as it surged.
Her campaign had its most embarrassing episode yet over the weekend when her party released her official platform to great fanfare, and then had to "correct" it not once but twice.
The changes withdrew support for gay marriage after preachers who share Silva's evangelical Christian faith criticized the platform, according to Brazilian media.
A statement supporting nuclear power was also removed. While neither issue is among Brazilian voters' main concerns, the gaffe reinforced views that Silva is volatile and disorganized. The revisions themselves were front-page news on Sunday.
In interviews, many Brazilians say they like Silva and would even tell pollsters they're inclined to vote for her. But many still haven't made up their minds, largely because of continued misgivings over her stability.
"She seems wonderful. I love her. But she doesn't inspire confidence," said Renato Nahoum, 31, a manager of a small supermarket in Sao Paulo.
Although Brazil fell into recession in the first half of this year, the story of the last 20 years has still been one of strong economic growth and falling poverty. Many with memories of hyperinflation and other chaos prior to the mid-1990s might be frustrated with Rousseff, but they're wary of big changes.
Two of Brazil's last five elected presidents were anti-establishment figures whose views were largely a mystery prior to taking office. Janio Quadros resigned after just seven months in power in 1961, while Fernando Collor quit amid corruption allegations in 1992. Comparisons between them and Silva have abounded on social media in recent days.
Meanwhile, newspapers have started punching holes in Silva's biggest strength - her claim to be a new and incorruptible kind of politician.
A front-page story in Folha de S.Paulo on Sunday said Silva had earned about $720,000 in speaking fees over the past three years. Other Brazilian politicians, including Rousseff's former chief of staff Antonio Palocci, have been felled by allegations they received legal pay in return for future access to power.
Media reports have also alleged the plane that crashed with Campos on board was acquired via precisely the kind of shady campaign donations that Silva often rails against. Her party vehemently denies the claims.
Finally, after a self-imposed period of restraint out of respect for Campos and concerns that attacking Silva could be counterproductive, her opponents are starting to take off their gloves.
In one of her most direct attacks yet, Rousseff said on Sunday she had read Silva's platform and was "very worried" by her proposals for industry, which she said could lead to mass layoffs by opening Brazil to greater trade.
(Additional reporting by Roberto Samora; Editing by Todd Benson and Kieran Murray)