Brazilians will decide Sunday between right-wing President Jair Bolsonaro and his leftist challenger, Luiz Inácio Lula da Silva, in what has become a nail-biter of an election.

Why it matters: Experts say Brazil's democracy — just over three decades old — is at stake.

• Brazil also has the largest gross domestic product in South America.

State of play: Most polls heading into Sunday give Lula a very slight advantage.

• But polls for the first round underestimated support for Bolsonaro, so it “will possibly be the closest race we’ve ever seen,” says Gustavo Ribeiro, journalist and analyst for The Brazilian Report. Lula won 48% of the vote in the first round, while Bolsonaro got 43%.
• Polls have tightened coming into the runoff. Analysts say that's in large part due to disinformation, misinformation and political polarization.

The big picture: Bolsonaro has repeatedly claimed without evidence that if he loses the election it will only be because of fraud, laying the groundwork for potentially denying the results.

• But Brazil’s electronic voting system has been in place since 1996 with no proven claims of tampering.
• Some experts fear violence will break out if Bolsonaro doesn't win. “If he loses, the risks are in how he will react,” Ribeiro says, adding that Bolsonaro could push for an uprising similar to the Jan. 6 insurrection in the U.S.
• A Bolsonaro ally was arrested last week after threatening a Supreme Court judge and throwing grenades at police.
• Researchers from the Federal University of Rio de Janeiro say reports of politically motivated violence increased 110% from July to just before the first round vote on Oct. 2.

What they’re saying: “This might be a sort of litmus test for the pursuit of democracy in Brazil,” Ribeiro says, adding that if Bolsonaro wins, the risk is a “backslide” to a less democratic country.

• Bolsonaro has praised the last Brazilian dictatorship, attacked the news media and undermined the nation’s Supreme Court.
• Lula is “flawed” but respects democracy, Ribeiro adds.

Flashback: Bolsonaro came to power in part because Lula was banned from running, due to corruption convictions which have since been thrown out, Axios' Dave Lawler writes.

• Bolsonaro's tenure has been defined by the pandemic, which devastated Brazil, and his constant fanning of culture wars.
• While many voters have fond memories of Lula's tenure, which coincided with a commodities boom, he and his Workers' Party are nearly as polarizing as Bolsonaro.
• Lula's platform is based less on specific policies than a promise that the good times will return.

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Editor's note: This story has been updated with additional details.