GABRIELA SÁ PESSOA MANAUS, Brazil — Environmentalists, Indigenous groups and climate scientists are celebrating the reelection in Brazil of President Luiz Inácio Lula da Silva , which brings the prospect of a continued focus on stopping the illegal cutting of the Amazon, the largest rainforest in the world.

But supporters also warned that a Congress increasingly dominated by the far right and the agribusiness caucus could undermine those efforts to protect the forest and combat climate change .

Lula defeated Sen. Flávio Bolsonaro, son of far-right former President Jair Bolsonaro, in Sunday’s runoff with 51.78% of the vote to 48.21%. After trailing in the first round, Lula staged a comeback to win by 4.4 million votes.

“The Amazon can breathe a sigh of relief!” said David Lapola, a Brazilian scientist and lead author of a chapter on so-called tipping points in an upcoming United Nations climate report. Scientists debate the threshold beyond which the dense, humid Amazon rainforest could shift to a drier ecosystem, with irreversible consequences for biodiversity and the climate. The forest absorbs vast amounts of carbon dioxide, a greenhouse gas that drives global warming. About 60% of the Amazon lies in Brazil.

Lula’s victory reflected shifts in voter support and higher turnout nationwide. In the Amazon, Indigenous communities and advocacy groups mobilized voters and, aided by court rulings last week, helped secure government transportation for those living in remote areas.

The Articulation of Indigenous Peoples of Brazil, known as APIB, was among the organizations that went to court to secure Indigenous voters’ access to the polls. Lula’s victory reflected the “strength of grassroots mobilization in defense of democracy, life and Indigenous peoples’ rights,” it said in a statement.

Flávio Bolsonaro refused to concede Monday and said he would ask the electoral court to nullify the results. He faces a high hurdle: presenting evidence of fraud sufficient to overturn Lula’s winning margin and persuading the court before the results are certified Dec. 18. Early signals from Brazil’s institutions and regional leaders, including conservative presidents who oppose Lula, suggest the outcome will be accepted at home and abroad.

Flávio Bolsonaro had pledged to pursue policies aligned with those of his father, who governed from 2019 through 2022. Deforestation surged under Jair Bolsonaro as his administration weakened environmental enforcement agencies and loosened regulations. Lula’s government last week announced the lowest Amazon deforestation rate on record.

Lula is expected to continue efforts to curb deforestation and finance a sustainable economy through international donations as well as public and private funding. While he was the clear favorite for environmentalists, climate scientists and others who want to protect the Amazon, Lula has put forward policies that have drawn criticism. Most recently, he has supported efforts to drill for oil in offshore waters near the Amazon.

Still, Flávio Bolsonaro presented a stark choice that for many environmentalists felt would put the Amazon and Brazil’s overall environmental protections at risk. He had denied climate change while casting himself as an environmental defender. However, he offered few details on forest protection or financing, calling for “modernizing” environmental laws and giving Amazon landowners more decision-making power — proposals environmentalists warned could weaken safeguards and accelerate deforestation.

Brazil’s Climate Observatory, a nonprofit coalition of 172 organizations, welcomed Lula’s reelection but said seven in 10 incoming lawmakers oppose the climate and environmental agenda. It cited recent measures weakening pesticide rules, Indigenous land rights and environmental licensing, and urged Lula to broaden his alliances and strengthen ties with Congress.

“The goal of ending deforestation across Brazil’s biomes must be pursued relentlessly,” the group said.

Amazon Watch, a nonprofit focused on environmental protection and Indigenous rights, also welcomed Lula’s victory while warning of legislative obstacles.

“A Congress strengthened by forces seeking to open Indigenous lands to mining and weaken environmental protections remains a serious threat,” said Ana Alfinito, an Amazon Watch legal adviser.

WWF Brazil said the country’s vast biodiversity and share of the Amazon give it a strategic role in addressing climate change. It credited Lula with strengthening Brazil’s leadership through international cooperation.

“Maintaining this leadership is essential to mobilizing other countries, raising the ambition of global commitments and turning agreements into concrete action,” the organization said.

Claudio Angelo, the Climate Observatory’s head of international policy, said Lula’s reelection had already delivered a result: a $500 million pledge Monday from the Netherlands to the Tropical Forest Forever Facility, a Brazil-backed fund launched at last year’s U.N. climate summit in Brazil .

The initiative pays developing countries to keep forests standing but has struggled to secure an initial $10 billion by year’s end. The Dutch pledge brings commitments to $7.8 billion.

Angelo said prospective donors had put plans on hold pending the election results, calling the Dutch pledge a vote of confidence in Lula’s victory.

“There was justified concern that a Flávio Bolsonaro victory could derail the TFFF, that Brazil would pull out and the fund would miss its $10 billion target,” Angelo said.