GEOFF BENNETT: This weekend, voters in Brazil head to the polls in a high-stakes presidential run-off that could reshape that country's political future.

Will Brazil join a rightward shift across parts of the Americas or stick with its left-wing leadership?

Brazil is South America's largest country, home to more than 200 million people.

Voting is compulsory for most adults there, and the race has deepened political divisions across the country.

Stephanie Sy has more.

STEPHANIE SY: The Rocinha favela is a sprawling, sloping community of 100,000 people overlooking a lesser-known part of Rio de Janeiro.

The serenity from an aerial view betrays the hardships of the working-class slum and the vicious political battleground it's become.

Brazilians will face a stark choice this Sunday, to either hand leftist President Luiz Inacio Lula da Silva a fourth term or replace him with Flavio Bolsonaro, the son of a former far-right president who was convicted for leading an attempted coup.

FLAVIO BOLSONARO, Brazilian Presidential Candidate (through translator): Brazil is without a president, but stay calm, because not for long.

Flavio Bolsonaro is coming.

LUIZ INACIO LULA DA SILVA, Brazilian President (through translator): I am convinced there is no reason why we shouldn't be elected president of the republic in the run-off.

There is no reason, economic, social, or political.

STEPHANIE SY: Both men are plagued by corruption claims, and both have their gloves off, as seen in this fiery debate.

At stake, the largest country in Latin America's political and economic direction, its relationship to the United States, and some say no less than the Amazon rain forest.

In the Rocinha favela, Brazil's divisions are laid bare.

Fabio Rosario, who's lived here for 40 years, says he's lost faith in 80-year-old Lula.

FABIO ROSARIO, Rocinha Favela, Rio de Janeiro, Resident (through translator): I realized it was all a lie, pure deceit.

He would say one thing and do another.

He was there for 16 years and didn't manage to accomplish anything.

STEPHANIE SY: But Tamara Moreira continues to support Lula for his social policies, which include cash transfers through his Bolsa Familia, or family allowance program.

TAMARA MOREIRA, Secretary, Rocinha Residents Association (through translator): As a mother, I need access to social benefits, health care, schools, programs like Bolsa Familia, and food assistance.

Lula's administration has always focused on the community, on single mothers and those with fewer privileges.

STEPHANIE SY: He's also focused on the rights of Brazil's indigenous tribes and protection of the Amazon, says Marina Silva, Lula's former environmental minister.

MARINA SILVA, Former Brazilian Environment Minister (through translator): Under Jair Bolsonaro's government, the Cerrado, which is one of the biomes most pressured today in terms of deforestation, saw a 47 percent increase.

He didn't create a single conservation unit.

He didn't demarcate any indigenous land.

STEPHANIE SY: But for the majority of Brazilians, this election is about the cost of living and violent crime.

Gilberto Thomaz works as a driver in Sao Paulo.

He backs the Bolsonaros, both father and son.

GILBERTO THOMAZ, Bolsonaro Supporter (through translator): When Jair Messias Bolsonaro came along, demonstrating that not everything in government was defined by corruption, we gained more power and strength to break away from what had been taught in schools, universities and elsewhere.

STEPHANIE SY: And like many on Brazil's political right, he sees the Bolsonaros' alignment with the Trump administration on what they call narco-terrorism as vital to clamping down on drug trafficking.

PATRICIA CAMPOS MELLO, Editor at Large, Folha de S.Paulo: And now we have the tightest contest ever in Brazilian history, electoral history.

I mean, it's virtually tied.

STEPHANIE SY: Patricia Campos Mello is editor-at-large at Folha de S.Paulo.

She says Sunday's election may prove more contentious than the 2022 contest, when Lula challenged Jair Bolsonaro.

Weeks after that election, Bolsonaro's supporters ransacked Brazil's presidential palace in a raid inspired by the pro-Trump activists who stormed the U.S.

Capitol.

While Jair serves a 27-year sentence for plotting a coup, his son Flavio has carried on the torch.

The 45-year-old former senator won a first-round vote on October 4 against Lula by 2 percentage points, setting up Sunday's run-off.

PATRICIA CAMPOS MELLO: What we do know is that never the candidate who finished second in the first round went on to win in the second round.

STEPHANIE SY: But Flavio's advantage is not assured.

Mello's investigation into the role social media may have played in the first round results has thrown a wrench into the race.

PATRICIA CAMPOS MELLO: The ad account of Lula Instagram was blocked.

They say it was a mistake, a technical problem due to a mistake.

But this was just for one hour.

And before that, there were reports of the views getting affected, but there's a lot of anecdotal evidence.

The only concrete things we have is the internal messages admitting to these problems.

STEPHANIE SY: Mello's report revealed that Meta, which owns Instagram and Facebook, experienced technical errors that may have disadvantaged Lula, including less visibility for his paid online ads.

Lula accused Meta of algorithmic manipulation, which the company denies.

This week, both candidates tried to have the other disqualified by a Brazilian court.

But the contest is still very much on, and the results should be known by late Sunday.

For the "PBS News Hour," I'm Stephanie Sy.