An estimated 33.4 million voters did not cast a ballot in Brazil's general election, the highest rate in decades. How might they affect the run-off?

Sao Paulo, Brazil – The first time she voted in a Brazilian presidential election, in 2014, Larissa Campelo voted for centre-right Senator Aecio Neves. Four years later, in the next presidential race, she supported a centre-left candidate, former Sao Paulo Mayor Fernando Haddad.

But ahead of what is expected to be one of Brazil’s tightest presidential run-offs ever, the 30-year-old mother of one has decided she has had enough with politics.

She is abstaining from the vote on Sunday, just as she did during the first round of the election on October 4.

Voting is compulsory for most Brazilian adults. But Campelo is part of a historically large chunk of the electorate sitting out this year's presidential race.

Some 33.4 million Brazilian adults did not show up to vote in the first round, resulting in a 21 percent abstention rate, the highest for a general election since 1998.

“It’s my form of protest. I don’t want to dirty my hands with these candidates,” said Campelo, who lives in the Sao Paulo city of Araraquara.

“This isn’t apathy. It’s because I don’t feel there’s anyone qualified enough to earn my vote.”But appealing to abstaining voters like Campelo may prove to be a path to victory in Sunday's run-off. The final vote features two presidential candidates well known in Brazil's political establishment.

The left-wing contender, Luiz Inacio Lula da Silva, is the incumbent president, seeking an unprecedented fourth, non-consecutive term in office.

His rival, Senator Flavio Bolsonaro, is a member of a far-right political dynasty, hoping to follow in the footsteps of his father, imprisoned former President Jair Bolsonaro.

In the first round of voting, Bolsonaro won 47 percent of votes cast, surpassing Lula’s 45 percent. Just 2.2 million ballots separated them, out of more than 158.7 million registered voters.

The final round is set to be even tighter. The research firm Quaest published a poll on Wednesday that showed the two candidates in a statistical deadlock.

Bolsonaro earned 50.5 percent of respondents' support, well within the margin of error for Lula's 49.5 percent.

But experts say polarisation within Brazilian politics has created a population of voters unwilling to cast a ballot for either Lula or Bolsonaro.

In the Quaest poll, 52 percent of respondents said they would never vote for Bolsonaro.

Some 47 percent said the same for Lula, although the pollster noted that the president’s rejection rate had fallen six points in a week, in a promising sign for the incumbent.

Joao Guilherme Bastos dos Santos, a director at the Democracy in Check Institute, an independent research nonprofit, explained that polarisation reinforces the public's disengagement with politics.

In a deeply divided political landscape, dos Santos pointed out that voter behaviour often changes.

Voters can become motivated by fear. In turn, they are more likely to vote in opposition to a candidate than in support of one.

"That creates profound exhaustion," dos Santos said. "The fighting seems to go nowhere." Campelo, the mother in Sao Paulo, echoed that sense of exhaustion. Her decision to stay home during Sunday's election was motivated in part by her rejection of Brazil's deeply antagonistic political culture.

"Politics in Brazil has become like supporting a football team," she said. "Politicians do things wrong, and people keep defending them. It makes no sense." The morning of her interview with Al Jazeera, Campelo said her husband's family broke into an argument over Lula on their WhatsApp group. "People are adopting their own pet politicians," she quipped.

Abstaining voters like Campelo are a diverse group, according to experts. Their reasons for dodging the ballot box can vary dramatically.

Mayra Goulart, a political scientist at the Federal University of Rio de Janeiro, said that those who actively reject Lula and Bolsonaro may only be a minority of Brazil’s abstainers.

Many could be merely uninterested in politics. "Life is difficult enough that they don't have the headspace, time or resources to devote to politics," she explained.

Luciana Veiga, a political science professor at the Federal University of the State of Rio de Janeiro, also pointed out that Brazil’s abstainers are not merely a bloc of disillusioned voters waiting to be won over.

They may have practical considerations that prevent them from casting a ballot. “We cannot say that abstention is simply a political 'no' to voting," Veiga said.

Abstention, she noted, often occurs among people who move cities without transferring their voter registration, as well as those who have work or family care duties on election day. There are also some voters in isolated areas who struggle to reach distant polling stations.

In addition, voting is optional for 16- and 17-year-olds, people aged 70 or older, and those who cannot read or write.

"Of Brazil's electorate of [158.7] million people, around 21 million are not legally required to vote," Veiga explained.

The penalty for failing to vote is often minor. Without justification for their absence, voters face a fine of up to 3.51 Brazilian reais — less than a dollar — and can have their registrations cancelled after three missed elections in a row.

Until these fines are paid, citizens are unable to obtain or renew passports and other ID documents, nor can they take part in the selection process for civil service jobs.

With such a slim margin between the candidates, both campaigns are hoping to court the first-round abstainers ahead of Sunday’s showdown.

In the weeks since the first vote, Bolsonaro has promised new tax breaks to help boost the economy.

A central part of his platform is a pledge to address his father's legal predicament: Jair Bolsonaro is currently serving a 27-year prison sentence following his conviction over a coup plot. Lula, meanwhile, has made reducing abstention a central part of his second-round push.

After round one, his campaign has highlighted Lula's work to end Brazil’s six-day workweek and unveiled proposals for free public transport on weekends and debt relief for small-scale entrepreneurs.

Goulart said that these measures "have much more of an effect on those who are uninterested in politics than on those who reject the candidates".

The first round, however, seemed to hand Bolsonaro an advantage. Not only did he secure the largest number of votes, but many of the candidates who were eliminated were his right-wing competitors.

Some experts predict that the nearly 8 percent who voted for those candidates will migrate to Bolsonaro's camp in the final round.

Bolsonaro also has historical precedent on his side. Brazil began holding direct elections for president in 1989, after its return to democracy. Since that time, no candidate who trailed in the first round of voting has ever won in the second round.

But Goulart warns against treating precedent as destiny. “A historical observation is not a causal relationship," she said.

She also pointed out that voters may act in unpredictable ways. Wednesday's Quaest poll suggests those who backed eliminated candidates in the first round have not uniformly flocked to Bolsonaro.

While the senator has attracted 43 percent of that support, Quaest found that 37 percent were leaning towards Lula — a more balanced redistribution than initially expected.

Quaest's results also indicate Lula may have an advantage among voters who abstained in the first round or left their ballot blank.

Forty percent of those respondents supported Lula in the poll, compared with 36 percent for Bolsonaro.

While poll numbers do not guarantee results, even modest fluctuations in voter participation could be decisive in such a narrow race, according to experts.

Veiga, the political science professor, believes some of the most crucial statistics will be voter participation rates in regions where each candidate has a strong base.

Lula has a left-wing stronghold in the northeast, while Bolsonaro boasts strong backing in the south and southeast.

"Look at whether turnout falls in either candidate's bases," Veiga said. "Any small movement makes a difference."