However the bespectacled 44-year-old senator, who is pitching himself as a more measured member of the far-right dynasty, has surprised many with a surge in opinion polls ahead of October elections.

"They always asked for a more moderate Bolsonaro, I've always been like this...balanced and level-headed," Flavio said after being tapped to lead his father's powerful conservative base.

Jair Bolsonaro, 70, has been barred from running and was jailed last year on charges linked to a failed coup attempt.

Flavio's presidential bid received a cool reception from conservatives and the market.

He also sparked confusion saying he might abandon the ticket for a "price," in what was seen as a bid to negotiate amnesty for his father.

But conservatives knew Flavio was not going anywhere and his ticket grew "even more than Bolsonaro supporters themselves believed," said political scientist Claudio Gonçalves Couto of the Getulio Vargas Foundation.

"I think it's the family brand that counts. This name (Bolsonaro) still has strength with a large segment of the electorate."

Leftist President Luiz Inacio Lula da Silva, 80, who beat Jair Bolsonaro by a razor-thin margin in 2022, is seeking a fourth term in the election.

Pollsters Datafolha said Flavio's support had doubled in the first round since a previous survey in December.

- A technical tie -

Their data, released Saturday, showed a statistical tie in the second round with Lula scoring 46 percent and Flavio 43 percent.

Back in December Lula led by 15 points.

Both men face high levels of rejection from voters in the sharply polarized nation.

"I don't trust the Bolsonaro family. In the second round, I would vote for Flavio Bolsonaro because I vote for any candidate who opposes Lula," said sales director Bruno Cayres, 38.

Critics associate Lula, who has served almost 12 years in office over three terms, with major corruption scandals and a failure to reign in rampant crime.

Supporters point to social welfare programs, and record unemployment under his government.

Opinion polls have shown that crime and violence are the leading concerns of Brazilians, followed by corruption and the economy.

Several names were floated to lead from the right, including Jair's third wife and former first lady of Brazil Michelle Bolsonaro -- who faced family criticism for lukewarm support of her stepson, Flavio.

Seen as a market-friendly favorite, Sao Paulo governor Tarcisio de Freitas has denied seeking the presidency.

Candidates will be confirmed by mid-August.

- 'A great deal of humility' -

Conservative lawmaker Evair de Melo said Flavio was "gradually introducing himself, including with a great deal of humility, acknowledging some of the mistakes his father may have made. This naturally humanizes his image."

The younger Bolsonaro, who was elected as a Rio de Janeiro deputy at 21, defends typical conservative positions, such as gun rights and Christian values.

During his time as a Rio legislator he was accused of involvement in a kickback scheme in which he took a portion of his aides' salaries, but the case was dismissed.

A hard-liner on security issues, he stirred controversy when he suggested on X that the US could bomb boats in Rio de Janeiro to fight drug trafficking.

In recent speeches, Flavio has focused on criticizing Lula's government while defending his father's legacy.

But he has tried to widen his appeal, with heartfelt videos on social media about issues affecting women, or posts about Brazilians going hungry.

In one post on X, he surprised many by using gender-inclusive language -- discouraged by his father's government which regularly slammed "gender ideology."

Flavio's press advisor said he would seek a woman running mate, possibly from the poor northeast -- Lula's stronghold.

Jair Bolsonaro's term in office was marked by controversy -- including comments slamming gays and demeaning women -- but he was praised for economic reforms and fiscal responsibility.

His 2019-2022 presidency is also largely remembered for rampant deforestation in the Amazon, downplaying the Covid-19 pandemic and questioning vaccines.

Flavio publically took the Covid shot.

"He is more educated, and trained in dealing with political elites and party backrooms," said Mayra Goulart, political science professor at the Federal University of Rio de Janeiro.

Flavio did not respond to AFP requests for comment.