Now Mr. Bolsonaro is betting on those same churchgoers to secure his presidential victory in the Oct. 30 runoff, spending much of his time on the campaign trail at sermons, shoulder-to-shoulder with the country’s most powerful preachers.

Wooed by his support for traditional conservative issues, some 60% of evangelical Christians have said they would Vote for him, compared with 32% for Mr. da Silva, according to a recent Ipec poll. If accurate, that would signal eroding support for Mr. da Silva, a former president who has seen his lead narrow to less than 5 percentage points in recent polls from more than 20 percentage points in May.

“Bolsonaro is the only one who is truly god-fearing among the lot of them,” said Francisneide Faustino, a single mother of four from Paraisópolis, a favela in São Paulo that is one of Brazil’s biggest.

Like many poorer Brazilians, Ms. Faustino said she has every reason to vote Mr. Bolsonaro out of office—she has gone hungry as food prices rose sharply during the Covid-19 pandemic and she just lost her job as a cleaner, one of some 10 million people unemployed in the country. But as an evangelical Christian, she will still vote for the man she calls Brazil’s “principled” president.

Among Brazilians who identify as Catholics, 56% said they would vote for Mr. da Silva, compared with 38% for Mr. Bolsonaro, although polls have shown that Brazil’s Catholics are less motivated by religious issues when they vote.

The number of evangelicals is expected to overtake the number of Catholics and become the biggest religious bloc in Brazil over the next decade. Low-income Brazilians, who polls show are more likely to be evangelical, voted en masse for Mr. Bolsonaro in 2018, creating a new pool of conservative voters.

No one knows for sure how big that pool is, political analysts say, partly because of delays in carrying out Brazil’s census.

While still identifying as Catholic, Mr. Bolsonaro, who was baptized in the Jordan River in 2016 and whose middle name Messias means “Messiah,” has cozied up to evangelicals since taking office in 2019 and has vowed to put “God above everyone.” His wife,

Michelle Bolsonaro,

is a devout evangelical.

From new tax benefits for churches to the appointment of pastors to government posts, including at least three cabinet positions, Mr. Bolsonaro has won favor with Brazil’s powerful religious leaders. Forbidden to campaign inside their houses of worship under Brazil’s electoral law, they have corralled votes on social-media platforms, which swelled during the pandemic as the faithful went online under lockdown.

Brazil’s 10 most popular evangelical preachers have a total of more than 100 million followers, according to a study in August by Casa Galileia, a democracy watchdog. While some evangelicals support Mr. da Silva’s effort to tackle poverty, “the majority see Mr. Bolsonaro as more aligned with their Christian values,” said Flávio Conrado, an anthropologist and researcher at the group.

Silas Malafaia, one of those most popular televangelists, who Mr. Bolsonaro took to London as his guest at Queen Elizabeth II’s funeral, is a fervent fan of the president.

“Bolsonaro will win these elections in the name of Jesus!” Mr. Malafaia said in a recent video posted to Facebook. He said Mr. da Silva had “no moral condition” to become president after being convicted of corruption in 2018 and jailed for more than a year.

Mr. da Silva, president from 2003 through 2010, asserts he is innocent of the corruption charge that resulted in the jail term and says he is the victim of a politically charged prosecution. He has struggled to fight back against the churches’ support for his rival. In power from 2003 to 2016, his Workers’ Party has backed socially progressive issues such as same-sex marriage and the legalization of abortion. To broaden his appeal, Mr. da Silva has worked to soften what some in Brazil consider his hard-left image.

In April, Mr. da Silva said that everyone should have the right to abortion, which is illegal in Brazil under most circumstances, calling it a public-health issue. After his comments provoked outrage among evangelicals, he backtracked and said he was personally against abortion, even though it disappointed many women in his own party.

“Lula believes in God and he is a Christian,” Mr. da Silva’s campaign team said in a statement this month, in which they also said, in response to accusations by Mr. Bolsonaro’s supporters, that “Lula does not have a pact with the devil, nor has he ever spoken to the devil.”

Though the Workers’ Party initially criticized Mr. Bolsonaro for bringing God into the presidential race, the party has now taken to citing the Bible in the campaign. Mr. da Silva is expected to publish an open letter to the nation’s evangelicals in coming days to reassure supporters of his commitment to Christian values, according to people close to his campaign.

The fight for the evangelical vote comes amid unfounded accusations on social media that Mr. da Silva is planning to close churches, pass pro-incest laws and publish a new Bible with a gender-neutral name for Jesus.

“Brazil’s tenuous secularism, which separates state and church, has been taken over by religious dogmas, fake news and misinformation, transforming the electoral debate into a type of holy war,” said José Alves, a leading demographer who has studied the rise of evangelicals.

Brazilians now give more credence to the opinions of religious leaders than party political campaigns on television or radio when they vote, according to a poll last month by Datafolha. Some two-thirds of Brazilians said the opinions of religious leaders influenced the way they voted, with half of those polled ranking it as a very important factor. Evangelicals gave even more importance to the church, with 75% saying religious leaders influenced their vote.

For months, pollsters had predicted a shoo-in victory for Mr. da Silva, whose presidency coincided with a commodity boom that lifted millions out of poverty. But Mr. Bolsonaro got 43.2% of the vote in the Oct. 2 first round, almost 10 percentage points more than some polls had predicted, although still 5 percentage points behind Mr. da Silva.

One of the biggest mistakes of polling firms such as Datafolha and Ipec was that they underestimated the number of evangelical Christians, said Mr. Alves, the demographer, who is also a former member of Brazil’s national statistics agency. While the firms based their results on the assumption that evangelicals make up 26% to 27% of the population, the real number is likely closer to 32%, according to Mr. Alves, who based his estimate on tax records showing the fast clip at which new churches have opened in recent years.

Ipec said that a new census would help to better estimate the number of evangelicals in Brazil, but that the data gives them no reason to believe it is higher than they have projected. Datafolha didn’t reply to requests for comment.

The election has caused a rift among some families. Camila Silva, an evangelical Christian and Bolsonaro supporter from  Minas Gerais, a vast inland state in the southeast, said her father is the only member of the family who supports Mr. da Silva, despite the best efforts of her mother and sister—also evangelicals—to get him to change his vote.

“It’s created a division here at home,” she said. “We’ve been trying to tell him that Lula is no good.”

Write to Samantha Pearson at [email protected] and Luciana Magalhaes at [email protected]