Brazilians are going to the polls on Sunday in a highly unpredictable runoff in which the leftist president, Luiz Inácio Lula da Silva, is attempting an unprecedented comeback against the far-right senator Flávio Bolsonaro, who won the first-round vote.

Bolsonaro’s surprise victory despite not leading most polls made him the favourite three weeks ago, but since then Lula’s campaign has gained ground, and the two go into the contest virtually tied.

The gap between them was the narrowest ever recorded in a first round, at less than 2% of the vote, and the 81-year-old president has spent much of the runoff campaign focused on turnout – abstention reached a record high in the first round and disproportionately affects his voters – and on stoking fears over what a return to power by a member of the Bolsonaro family could mean.

A congressman for more than 20 years with few bills passed, the 45-year-old Bolsonaro is the eldest son of the former president Jair Bolsonaro, who is under house arrest serving a 27-year sentence for attempting a coup to overturn his 2022 defeat to Lula.

The senator has promised that if elected he will pardon his father, and with an even more conservative congress taking office next year he has vowed to “change the constitution and re-democratise this country”, sending chills through those who fear he could follow in his father’s authoritarian footsteps.

“We’re facing two completely different projects for the country,” said voter Nuno Rau, 63, an architect and writer. “One points towards the end of social rights and the end of democracy itself; on the other side is someone who fought for income distribution and free public education. I may have criticisms of Lula, but there is no way to choose Bolsonaro,” he added.

Aparecida Moreira, a 62-year-old lawyer, will vote for Bolsonaro mainly because of the “iron fist” security policy he proposes, including building five “mega-prisons” inspired by those of El Salvador’s populist autocrat Nayib Bukele.

“I agree with Bolsonaro’s idea of chemical castration for rapists, because there is no crime worse than rape,” said Moreira, adding that voting for Lula would be the same as voting for the “dictator” Alexandre de Moraes, the justice who, despite not having been appointed by Lula, is widely associated with him by Bolsonaro supporters because he oversaw the former president’s conviction.

Both Flávio Bolsonaro and Lula drew large crowds to their final campaign rallies on Saturday in Rio de Janeiro, São Paulo and Belo Horizonte.

With each side of supporters virtually “crystallised”, the election will be decided by the small number of voters who identify with neither candidate.

Bolsonaro began the runoff campaign with a lead and the support of most other defeated candidates, but instead of growing, he stagnated in the polls while Lula gained support.

Many analysts saw this as the result of part of the population suddenly waking up to the “fear” of a Bolsonaro returning to power, as well as controversies including the fact that the senator is under investigation in two federal police inquiries over alleged corruption and money laundering.

Lula, by contrast, radically changed his digital strategy and benefited from student-led protests across the country, most of which did not even involve the president.

“In the first round, Lula’s campaign failed to connect with the population, but now it has been civil society movements that have driven his growth,” said the political scientist Helcimara de Souza Telles.

Whatever the result, the race is expected to be decided by a narrow margin in Latin America’s largest country, with an electorate of 159 million.

Many analysts believe that if Lula overturns his first-round defeat, Bolsonaro could follow his father’s example from four years ago and refuse to concede, and that Donald Trump, a longstanding ally of the Bolsonaro clan, could refuse to recognise the result.

“Trump wants to ‘colonise’ all of Latin America in pursuit of its own interests,” said Telles, adding that, with rightwing candidates having won all seven presidential elections held in Latin America since Trump began his second term, Brazil is one of the few countries in the region still offering any resistance to the US president.

“So a Bolsonaro victory would mean losing Brazil to a model of autocracy backed by the US,” she added.