Brazil's election on Sunday could spell the end of Jair Bolsonaro's presidency — or set the stage for a very tense four weeks leading up to a runoff with leftist former President Luiz Inácio Lula da Silva.

State of play: With Lula leading by at least seven points in all the latest polls, the big question is whether the former president can secure an outright majority.

• If neither candidate hits 50%, as looks likely, the two will square off again on Oct. 30. While a late charge from Bolsonaro can't be ruled out, Lula would be heavily favored.
• But Bolsonaro has claimed without evidence that Brazil's electronic voting system can be manipulated, and said in recent days that if he receives less than 60% of the vote "something abnormal has happened." The far-right firebrand once famously declared that his tenure would end in one of three ways: victory, prison or death.
• A string of senior Biden administration officials have tried to pre-empt an electoral dispute by publicly praising Brazil's voting system and raising the issue with Bolsonaro behind closed doors. The U.S. Senate last night signaled its concern with a resolution led by Sen. Bernie Sanders (I-Vt.) in support of Brazilian democracy.

The big picture: If Lula is declared the winner, Bolsonaro will almost certainly challenge the result in his rhetoric and potentially in the courts, says Thiago de Aragão of the Brasilia-based Arko Advice consultancy. But de Aragão believes fears of a coup to keep Bolsonaro in power are overblown.

• Unlike Donald Trump after the 2020 election, Bolsonaro doesn't have a strong political party behind him in Congress. And despite his administration's close links to the military, the generals have no intention of stepping into the political arena, de Aragão contends.
• While Brazilian democracy was only restored in the late 1980s (following a military dictatorship much-praised by Bolsonaro), the country has relatively strong institutions and a clear separation of powers.
• Still, Beatriz Rey of the Center for Studies of the Brazilian Congress expects "chaos" if Bolsonaro loses. She said at a recent Wilson Center event that political elites may feel the need to make a post-election deal with Bolsonaro to convince him to leave office.

Meanwhile, a series of politically motivated attacks have increased the sense of foreboding leading up to the vote.

• This past Saturday, a Bolsonaro supporter allegedly entered a bar, asked who planned to vote for Lula, and fatally stabbed a man who said "I will." In July, another Bolsonaro supporter allegedly shot and killed a man who was holding a Lula-themed birthday party.
• Violence has broken out at rallies for both parties. Bolsonaro was himself stabbed while campaigning in 2018.

State of play: Bolsonaro came to power in part because Lula was banned from running, due to corruption convictions which have since been thrown out. 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.
• The theme of this election is "rejection," de Aragão says. Most voters will be casting their ballots against Bolsonaro or Lula.
• Lula, who turns 77 in October, has led consistently in the polls. But his platform is based less on specific policies than a promise that the good times will return. Lula tends toward pragmatism rather than ideology on economic issues, de Aragão notes.

What to watch: In international affairs, however, Lula could position himself as a leader of the growing club of left-wing leaders in the region.

• Lula has also advocated close economic links with China and blamed NATO for Russia's invasion of Ukraine during the campaign.
• So while relations between presidents Biden and Bolsonaro have been awkward — particularly on Biden's core priorities of climate and democracy — a Lula victory would bring challenges of its own for Washington.

Editor's note: This story has been corrected to note that Lula turns 77 in October, not this month.