Marina Silva campaigning for president in Rio de Janeiro last week. Associated Press
RIO BRANCO, Brazil— Marina Silva started life as the child of illiterate rubber tappers trudging malarial jungles to collect latex sap in a remote corner of Brazil's Amazon. She was baptized in politics confronting gun-toting cattle ranchers sent to burn down her forest home.
Now, the 56-year-old activist-turned-senator may become president of Latin America's biggest nation.
Polls show Ms. Silva would win enough votes to force a runoff against incumbent President Dilma Rousseff in Brazil's Oct. 5 election, and win in the second round three weeks later. It is a major shift from a few weeks ago, when Ms. Rousseff appeared headed for re-election and Ms. Silva wasn't in the race. But Ms. Silva has soared in the polls since replacing a Socialist Party candidate killed in a plane crash last month.
The surging Silva candidacy carries enormous unknowns for the world's seventh biggest economy, now coming to grips with the end of a yearslong commodity boom. Ms. Silva's supporters say she is the reformer Brazil needs to stamp out cronyism and other political ills that are stifling development. Critics see her as an outlier who would be chewed up by Brazil's Darwinian multiparty system, adding even more uncertainty to the outlook for the emerging economy.
"She is riding a great wave of disenchantment with a political system that everyone agrees needs reforming, but there are real questions about whether she would have the conditions to actually govern the country," said Tereza Cruvinel, a political analyst and writer based in Brasília. (Read more about the leading candidates.)
Ms. Silva's candidacy is channeling voter anxiety amid a jarring reversal of Brazil's prospects. In 2010, the economy raced forward at 7.5%, sparking optimism Brazil was making a critical development leap. But the economy has stagnated ever since and slipped into recession this year. In 2013, around a million Brazilians took to the streets to protest everything from corruption to poor hospitals and the $11.5 billion price tag to host the World Cup.
Ms. Silva's outsider résumé and promise of a "new politics" has fueled hope among voters seeking change. She would be Brazil's first black president, a landmark for a country that was the last to abolish slavery in the hemisphere in 1888. Her climb from poverty—learning to read as a teenager and working once as a maid—resonates with Brazil's poor. A devout evangelical Christian, she would be the first Protestant leader of the world's largest Catholic nation. She entered politics in the Amazon at the side of rain-forest activist Chico Mendes, whose 1988 assassination helped spur the global environmental movement.
"The old structures are not going to win," Ms. Silva said during a Sept. 1 televised debate. "It's important that each Brazilian doesn't give up their hope, because there is a big effort on now to make you recoil into fear."
Marina Silva Zuma Press
Ms. Silva's spokesman said she was campaigning and wasn't available for an interview.
The fact that Ms. Silva is able to run an antiestablishment campaign against a leftist Workers Party candidate underscores how fast Brazil's political landscape has changed since it embraced democracy in 1985. Ms. Silva was a member of the Workers Party, an opposition party that also led strikes and protests to force a military dictatorship to loosen its grip.
But the party has been in power for 12 years now, a stretch marred lately by corruption scandals, economic stagnation and rising inflation. Ms. Silva broke with the Workers Party in 2009.
President Rousseff's campaign is hitting back. A series of attack ads paint Ms. Silva as a politically isolated dreamer in the mold of past Brazilian presidents who were felled by coups or impeached.
"Dreaming is good but in elections you should vote with your feet on the ground," Ms. Rousseff says in one ad. "A government without support in congress is not going to be able to ensure a stable government, a government without institutional crises."
Ms. Silva, front, as an environmental activist in the Amazon in a circa-1980s photo. Biblioteca da Floresta (file photo)
A former Marxist guerrilla, Ms. Rousseff is seeking to close the gap by nurturing support among the millions of rural poor who have benefited from welfare expansion under Workers Party rule. Rousseff supporters say Ms. Silva is too close to bankers.
But the focus of the Rousseff campaign has become attacking Ms. Silva as unfit for the job. Ms. Silva would take office atop a small political party with a tiny minority in a fragmented Congress split between three big parties and constellations of tiny ones. Two of the large parties are firmly in the Rousseff camp.
To govern, she would need to cobble together a majority through careful consensus building. But Ms. Silva has often walked away from negotiations rather than compromise her ideals—a trait seen as an asset by supporters and a liability by critics. Ms. Silva's Socialist Party run marks her fourth political party in five years. A Brazilian magazine ran a spoof article about the serial breakups under the headline "Marina Silva breaks with Marina Silva."
Ms. Silva's ability to keep her own diverse coalition together is already being tested. She angered some supporters by backing away from a gay-rights plank she announced only days earlier after coming under fire from evangelical conservatives who oppose gay marriage.
Former President Fernando Henrique Cardoso recalled how his own efforts to gauge her backing for his more conservative Social Democrat party ran aground in 2010 during a three hour conversation after Ms. Silva was ousted from that year's presidential contest. "My conclusion was that she would never support anyone, because she believes that she's predestined to be president," Mr. Cardoso said.
On that occasion, Mr. Cardoso said, Ms. Silva asserted that if she were to be elected president someday she would be able to form a successful government drawing on "good people" from rival parties.
For now, Ms. Silva is taking votes from both Ms. Rousseff and from supporters of the conservative candidate Aécio Neves, who are looking for any candidate who can beat Ms. Rousseff. She is also getting a big share of voters so fed up with politics they were planning to submit blank ballots, polls show.

Marina Silva: From Rubber Tapper to Brazil's Presidential Candidate

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Above, Marina Silva in 1983 at the Catholic school where she taught history in Rio Branco. She came to Acre's state capital from the rain forest as an illiterate, 16-year-old rubber tapper, hoping to enter the convent. She attended Catholic school and learned to read. In the late-1970s she became an environmental activist, joining Chico Mendes to protest ranchers who were burning down the Amazon to create pastureland. Immaculate Conception Institute
Still, Ms. Silva has only been in the race a matter of weeks, and her momentum could slow. Running as vice presidential candidate on the Socialist Party ticket under Eduardo Campos, she was propelled into the race after Mr. Campos's campaign plane crashed near São Paulo on Aug. 13.
Ms. Silva enjoyed a burst of mostly sympathetic media exposure, including cover-page profiles in national magazines, after the crash. As the election draws nearer that coverage may turn into a more critical look at her platform.
Ms. Rousseff can deploy considerable firepower to hammer Ms. Silva. Brazilian election rules favor incumbents, and Ms. Rousseff gets about 12 minutes of nightly free-TV time compared with less than two minutes for Ms. Silva.
The diverse groundswell of support lifting Ms. Silva was on display Aug. 29 as she unveiled her platform to an unusual mix of environmentalists, left-leaning progressives and conservative financiers crammed into a ballroom in São Paulo. A solemn national anthem was followed by boisterous soccer-style cheers.
"Everyone is closing their eyes and jumping on board. I'm not worried about her economics, since nothing can be worse than it is now. I'm here to figure out the politics," said an economic analyst at the event.
Ms. Silva's platform mixes orthodox economic measures, such as reducing debt and price controls and creating a U.S.-style independent central bank, with a heavy dose of environmentalism, such as adding more wind power to Brazil's energy mix, enforcing regulations and factoring in the environmental costs to big building projects.
But a central plank of Ms. Silva's program would be to reform a political system where presidents buy votes by giving control of key government agencies to minority parties, according to Maria Alice Setubal, an heiress to the Itaú banking fortune who is a key backer.
To introduce such major reforms, Ms. Silva may call national votes to get support for key ideas that meet political resistance in congress, supporters say.
"Marina has always created very strong bonds with the people wherever she is, and I believe she will use this power to avoid becoming a hostage to the parties in congress, the interest groups," said Julio Pereira, a Rio Branco physician turned politician who was Ms. Silva's handpicked replacement for her vacant senate seat.
Ms. Silva is hardly a political neophyte. She came in third in the 2010 election with around 20 million votes on the Green Party ticket.
And Ms. Silva is no stranger to overcoming hardships, her supporters say.
She was born in the Amazonian state of Acre, a corner of the Brazilian rain forest that was all but cut off from the world in the 1960s. In her biography, Ms. Silva recalls her family seeing photos of John F. Kennedy's assassination in a magazine for the first time in 1968, five years after it happened.
The Silvas were part of a wave of mostly black and poor Brazilians lured to the Amazon to tap rubber by the government, but who found hardships such as debt slavery and disease. They lived in spare dwellings in remote camps often a day's walk from the nearest dirt road. While still a child, she began her days well before dawn, cooking over wood fires before making a long round trip walk to collect latex.
Her early years permanently marked her world view and are a foundation of her environmentalism.
"Living in the forest gives you a different perspective, you see the world in a more humane, connected way. Marina has this," said Elson Martins, an Acre based-journalist who also grew up in rubber camps and participated in the fight against deforestation with Ms. Silva and Mr. Mendes in the 1970s and 1980s.
Of 11 siblings, three died, two of malaria. Ms. Silva suffered so many bouts of malaria, hepatitis and other tropical afflictions such as leishmaniasis that a doctor once declared her to be dying.
She took on more responsibility after her mother died of an aneurysm, according to her younger brother Antonio Arleir Silva, 48.
"I remember having malaria and Marina caring for me, telling me I was going to make it, when she was suffering malaria herself and didn't show it," Mr. Silva said.
One source of her strength was God, family members say. Ms. Silva left the rubber camps for Rio Branco, the state capital, as a teenager to recover from disease and pursue a dream of becoming a nun. She later left the Catholic Church, and is today a member of evangelical Assemblies of God.
Ms. Silva's faith attracts support among Brazil's growing population of evangelicals. At the same time it can seem incongruous to secular supporters drawn to her environmentalism.
"God is one of her key advisers, and sometimes that means we need to be patient," said a Silva campaign official.
Ms. Silva consults the Bible daily, people who have worked with her say. She has used a "Bible roulette" technique to help make decisions that involves opening up the Bible at random and searching for meaning in whatever passage is on the page, according to her biography, "Marina: Life for a Cause."
As an illiterate teenager in Rio Branco, she worked as a maid for the family of Terezinha da Rocha Lopes, 82, cooking and cleaning in exchange for room and board. It was family of eight children, mostly shoeless, living in a three-room, wood-plank house built on stilts. Ms. Silva slept in the same bed with one of the Rocha Lopes girls. They made pretend altars and saints and prayed to them for fun.
Ms. Silva entered the convent, where she finally learned to read. Eventually, she left religious training, feeling a call to activism.
Starting in the 1970s, poor rubber tappers in Acre were banding together against ranchers who were burning down the forest where they lived to turn it into cow pasture. Led by Mr. Mendes and others, the tappers would stand in human chains to prevent the rain-forest destruction. Mr. Mendes recruited Ms. Silva into the struggles.
Ms. Silva had personal reasons to join. The invading ranchers, backed by the country's dictatorship, were burning down forest around the Silva home.
It was dangerous work. Mr. Mendes and other leaders were killed. But their work was largely successful: Today Acre remains around 90% forested. When Brazil's dictatorship fell, Ms. Silva was among the union leaders, left-wing Catholics and intellectuals who joined the nascent Workers Party to channel their activism into government.
Ms. Silva won a spot as city councilwoman in Rio Branco in the late 1980s, eventually climbing to the Senate in 1994 and to a stint as Environment Minister from 2003 to 2008 in the Workers Party government of President Luiz Inácio Lula da Silva. In the city council post, she made waves by trying to return to city coffers a bonus payment the council had awarded itself. A city ordinance was written to prevent her from giving it back. So Ms. Silva spread the money out among groups pushing for rain forest protection and open democracy.
As environment minister she introduced policies such as heavier enforcement and monitoring that are now credited with helping reduce Brazilian deforestation rates to their lowest ever. Global environmental groups now say a Silva presidency would strengthen international efforts to combat global warming.
But it could also promote fierce battles with Brazil's vast agribusiness, power and construction sectors.
In a prologue to this year's election, Ms. Silva battled with Ms. Rousseff over Amazon development policy while the two women were cabinet ministers in a previous Workers Party government. Ms. Silva eventually quit over plans to string the Amazon's big rivers with giant hydroelectric dams.
On the campaign trail, traces of the illiterate young rubber tapper are hard to find. Ms. Silva is today better known as a voracious reader who often emerges from her office with a book still in hand. She writes poetry and sprinkles her speeches with references to writers and philosophers. A quote that she often cites lately from the 19th-century French poet Victor Hugo has become a campaign theme for some advisers: "One cannot resist an idea whose time has come."
—Reed Johnson and Rogerio Jelmayer contributed to this article.
Write to John Lyons at john.lyons@wsj.com