The winner of the election will likely play a major role in deciding the fate of the Amazon rainforest.

The Amazon rainforest in Brazil has experienced sharply increasing levels of illegal deforestation and mining, violence against environmental activists and Indigenous people and a loosening of environmental enforcement under the administration of President Jair Bolsonaro.

The future of the rainforest became a pressing issue as Brazilians headed to the polls in Sunday’s presidential runoff.

“Candidates, whether they like it or not, are having to express their views about the Amazon,” said Natalie Unterstell, the head of Talanoa, a climate policy research institute in Brazil. “The Amazon places Brazil in world geopolitics. It is where we make a difference.”

The destruction of the Amazon rainforest, the world’s largest rainforest with 60 percent of its area inside Brazil, poses a threat to the Earth’s climate since the Amazon helps keep tens of billions of tons of carbon dioxide out of the atmosphere.

Since taking office in 2019, Mr. Bolsonaro championed industries driving the rain forest’s destruction, loosened regulations to expand logging and mining in the Amazon and scaled back environmental protections. He also slashed federal funds and staffing, weakening the agencies that enforce Indigenous and environmental laws.

Yet, in addressing world leaders last month at the United Nations General Assembly, Mr. Bolsonaro said that more than 80 percent of the Amazon is untouched and remains home to more than 20 million Indigenous people. He also told the Assembly that Brazil was a “reference for the world” in the protection of the biosphere.

For his part, Luíz Inácio Lula da Silva promised to stop deforestation and illegal mining if elected president, as well as open discussion of a different economic model for the rainforest.

“I want to take care of the Amazon, not make it a sanctuary for humanity, but through research and partnership with other countries, explore its biodiversity,” Mr. da Silva said recently in a radio interview.

Even after the elect ion, disputes on the future of the biome will keep pressing the capital, Brasília. On Oct. 2, Brazilians elected several members of Congress who may push the frontier of agribusiness and mining farther into the rainforest.

Ricardo Salles, Mr. Bolsonaro’s former environmental minister who resigned after he was accused of being involved with illegal logging, was elected to Congress, though he still faces charges related to the case.

A major advocate for greater protections of the Amazon, Joênia Wapichana, the only Indigenous representative in Brazil’s capital, was not re-elected. However, the election did see two new Indigenous representatives — Sônia Guajajara and Célia Xakriabá — win their races.

“The same hand that holds the chain saw at the forest is the hand that allows Congress to kill our rights,” Ms. Xakriabá said in an interview. “More than 250 bills rolled back environmental and land protection, and we will fight against them.”

Ms. Unterstell said much remained to be done to address the issue of climate justice to help those, like Indigenous people, who are threatened by worsening environmental conditions.

“The next government will define the fate of the forest,’’ Ms. Unterstell said.