It had been nearly a decade since Celia de Oliveira Soares last voted, and months since the 101-year-old great-grandmother last left her home.

But on 25 October, when Brazil held what many considered its most important presidential election in decades, relatives helped Soares into a wheelchair and drove her to a nearby school to make sure the far-right candidate, Flávio Bolsonaro, did not prevail.

Soares clung to the banister as, step by step, she climbed two flights of stairs to the polling booth under the watchful eye of her granddaughter's husband, Marcio Marques.

Inside, the frail but jaunty centenarian keyed the number "13" – representing Bolsonaro's leftwing rival, President Luiz Inácio Lula da Silva – into an electronic voting machine and pressed confirm.

"I like Lula. I really do!" enthused Soares, a retired bank employee who has lived in Rio de Janeiro most of her life.

"I don't like Bolsonaro or his father. So the efficient thing to do was go out and vote," Soares added, as her grandsons, Tito and Ravi, played on the balcony around her surrounded by pot plants and books.

Soares is one of more than 1 million elderly people who, having missed the election's 4 October first round – which Lula lost – decided to vote in the second.

Never since the return of democracy in the 1980s had the first-round loser bounced back, but Lula did just that. And after Lula's shock revival, elderly voters have been hailed as the election's unsung heroes – including by Lula himself.

At his 81st birthday party last week, the leftist veteran shed tears while describing his gratitude to the elderly people who helped him clinch a historic fourth term while clutching walking frames and sticks.

"I honestly think it was the most powerful and human image I've ever seen during an election," Lula said of viral pictures showing voters in their 80s, 90s and 100s being carried into polling centres by relatives or carers, some in stretchers or wheelchairs.

In north-east Brazil, where Lula was born into rural poverty in the 1940s, a 114-year-old retiree called Josefa Pereira da Conceição went to vote wearing a red T-shirt emblazoned with Lula's face and the slogan "Strength of the people".

Election experts are still crunching the numbers to fully understand how Lula pulled off his unprecedented fightback, finding 10.3m extra votes between the first and second rounds.

Explanations include a sharp increase in welfare benefits on the eve of the vote; an extraordinary digital mobilisation involving a pro-Lula WhatsApp group that grew from 50,000 to nearly 11m members in less than a month; and a student-led street rebellion in which millions of citizens protested across Brazil.

Some believe elderly people were also crucial, noting how 1.4 million of the 3 million extra voters who took part in the second round after skipping the first were over 70. Pre-election polls suggested such voters were more inclined to vote for Lula.

Speaking at Lula's victory celebration, the former minister Simone Tebet called the result a cross-generational triumph which sent a message to the democratic world. "It is possible to resist fascism – all you need to do is mobilise society," Tebet declared, claiming: "Young people hit the streets and golden-agers saved our democracy."

The political journalist José Roberto de Toledo said there was concrete evidence of higher turnout among older voters in the runoff, particularly those aged 60 to 79. Voting is only mandatory in Brazil for those aged 18 to 69.

"The images [of elderly people voting] aren't misleading: they reveal a phenomenon that isn't just an isolated case. It's not some made-up story or a staged example. It really happened – and it was a movement you can measure statistically," Toledo said of the images suggesting an unusually large number of older voters.

That said, Toledo believed the swing to Lula was explained mostly by fear, not age. Voting data suggested many people who backed Lula in the runoff had previously voted for Bolsonaro's rightwing adversaries, hoping to protest against Lula's Worker's party (PT), which has governed for nearly 18 of the last 24 years.

In the runoff, faced with a straight choice between Lula, a familiar and predictable figure even for detractors, and Bolsonaro, who represented a leap into the unknown, many preferred the former.

"A protest vote in the first round [became] a safety vote or a lower-risk vote in the second – even if it wasn't a happy vote," said Toledo, who believed Bolsonaro alienated many voters with his "arrogant" behaviour after winning the first round.

Particularly damaging was the rightwinger's authoritarian-sounding vow to "re-democratise" the already democratic country – something Toledo suspected stoked fears that Bolsonaro aspired to be "the emperor of Brazil".

Even before those remarks, Soares fretted over the direction Brazil might take under Flávio Bolsonaro, whose father, the former president Jair Bolsonaro, is serving a 27-year sentence for plotting to seize power after losing the 2022 election to Lula.

"His father wanted to stage a coup … and stay on as president, just like Getúlio [Vargas did in 1937]," recalled the great-grandmother, who was 12 at the time and lived through the subsequent eight years of dictatorial rule.

Marques, a 51-year-old food entrepreneur, said his grandmother-in-law was also critical of how Jair Bolsonaro undermined vaccination efforts during the Covid pandemic, when 700,000 Brazilians died.

"She's got three great-grandchildren … and she understands she's nearing the end of her life. But she's worried about the country, about her family and about future generations," said Marques, becoming visibly emotional as he recalled watching Soares exercise her democratic right, perhaps for the last time.

"It was so moving to see her strength and desire [to vote] … We felt she had an intuition, or something telling her she needed to be part of this … She doesn't know if she'll get another chance," added Marques, whose 85-year-old father, Helço Muniz, also voted Lula – in his case for the first time.

Sitting in her armchair overlooking Rio's Guanabara Bay, five days after the election, Soares voiced excitement that Lula would soon start his fourth and final term. "I've been fond of him ever since he was poor," she said of the lathe operator turned president, who first took office as president in 2003. "He was poor and he became rich, but he carried on being a simple man."

Occasionally, Soares's memory betrayed her and she needed reminding that Lula had pulled off one of modern history's greatest democratic comebacks. "But that's wonderful!" she beamed.

Reminded of Lula's feat, Soares predicted Bolsonaro would flee abroad.

"He's already gone!" her granddaughter, Poliana Soares Jordão, 46, told her. "His dad's in prison and he's fled."

Soares, who has lived through four coups, 16 presidential elections and four Lula wins, was not surprised. "He must have gone to the US, then," the centenarian guessed, correctly. "He'd been getting ready for that for a while, hadn't he?"