Thundering to a crowd of thousands of far-right supporters in São Paulo on 7 September, Brazil’s Independence Day, the senator Flávio Bolsonaro focused less on his opponent in the presidential election, the leftist president Luiz Inácio Lula da Silva, than on someone who is not running at all.

“Whoever votes for Lula is voting for Alexandre de Moraes,” Bolsonaro shouted, referring to the shaven-headed supreme court justice once described as “Voldemort” by Elon Musk.

Although Lula did not appoint Moraes to the court, Bolsonaro’s supporters have linked the two men ever since the judge oversaw the case in which the senator’s father, the former president Jair Bolsonaro, was sentenced to 27 years in prison for attempting a coup.

Now, just days before the election runoff on 25 October, Moraes is once again at the centre of a presidential scandal – one that Lula believes may have helped Flávio Bolsonaro overtake him to emerge as the frontrunner. The justice once hailed as democracy’s “saviour” for containing Jair Bolsonaro may now have helped pave the way for the family’s return to power.

The scandal began with a police investigation, which found that Moraes’s wife’s law firm had a contract worth about £20m with Daniel Vorcaro, the banker accused of orchestrating Brazil’s largest banking fraud. Text messages also emerged suggesting a close relationship between Moraes and Vorcaro, though the judge denies the messages are authentic.

Investigations subsequently revealed that at least four other justices had links to the disgraced banker, meaning half of the court’s 10 current justices have been drawn into the scandal.

The crisis peaked last month with a ferocious exchange of insults between Moraes and another justice during a televised court session. A series of rulings followed in which the pair – as well as other judges aligned with each side – overruled one another, dominating public debate for weeks.

Lula, the day after losing the first round, blamed his poor result in part on having to “carry on his back … the promiscuity” of the supreme court, in what many political observers saw as an attempt by the president to distance himself from the crisis. His campaign coordinator, Edinho Silva, said that, “in the eyes of society … [Lula] paid the price for that institutional damage”.

Although analysts point to a range of reasons for the far right’s lead, from household debt to a simple preference for Flávio Bolsonaro, many agree that the supreme court crisis played a decisive role, particularly among indecisive voters who, with the electorate generally divided between two similarly sized left and right blocs, are the ones who ultimately decide the election.

“These are people who thought that, just as they see congress, the country’s highest court had become corrupt, too. That deepened apathy among some who might otherwise have voted for Lula and fuelled the anti-system sentiment that feeds the far right,” said Camila Rocha, a political scientist who studies Brazil’s new right and has spent recent months interviewing undecided voters.

“We asked people about the court crisis, and they would say: ‘Everything is very confusing, but one thing we do know is that the big villain is Alexandre de Moraes,’” she said. “Precisely the judge who jailed Jair Bolsonaro now emerges, for these people, as the big corrupt figure and the main villain.”

Conrado Hübner Mendes, a professor of constitutional law at the University of São Paulo and a leading scholar of the supreme court, said Moraes put himself in that position by “embracing the role of heroic protagonist”.

The judge blocked some of Jair Bolsonaro’s moves during his government and drew the ire of far-right figures such as Musk and Donald Trump. The US went as far as to impose sanctions on him, in a move widely seen as an attempt to interfere in Jair’s trial.

On Monday, Moraes authorised Flávio Bolsonaro to resume visiting his father after a 90-day ban imposed in July, when the senator read out a letter from his father about the elections that Moraes considered a breach of Jair Bolsonaro’s ban on using social media.

Beyond the anger of Bolsonaro’s supporters, legal scholars and some on the Brazilian left, while acknowledging Moraes’s decisive role in the coup trial, also criticised several of his other rulings as authoritarian.

“He took on a huge responsibility, and now he has thrown that away in the most mediocre way possible, with this alleged corruption case,” said Mendes.

While acknowledging the contract between the law firm run by his wife, Viviane Barci de Moraes, and the disgraced banker – who is worth an extraordinarily large sum by Brazilian law firm standards – Moraes denied any wrongdoing. He said he had never ruled on any case involving Vorcaro and that there was therefore no conflict of interest.

Vorcaro has been behind bars since he was arrested in November as he prepared to board a private jet from São Paulo to Malta. He is accused of leading a scheme through his Banco Master involving aggressive fundraising, unrealistic returns and falsified balance sheets that left an estimated £9bn hole, much of it involving public money and pensioners’ savings.

Text messages extracted from Vorcaro’s phone show that, two days before his arrest, the banker asked someone whom police later identified as Moraes: “Do you think … I need to get out [of the country]?” Moraes denies being the recipient and accuses a fellow justice, André Mendonça, appointed by Jair Bolsonaro, of using the police against him for electoral reasons, allegedly to benefit Flávio’s campaign.

Mendonça is one of the four other justices suspected of having links to Vorcaro. An institute he created is said to have taken out an £800,000 loan from one of the banker’s associates, though he says it has already been repaid. In the other justices’ cases, investigators uncovered links ranging from a £64m contract to an alleged stake in a fund that received £5.2m from Vorcaro. All deny wrongdoing.

No reports had linked Lula to the scandal until Friday, when Veja magazine reported that Vorcaro allegedly said he had paid £4.5m to finance a 2024 Oliver Stone documentary about the president. The federal police and the public prosecutor’s office, however, said the banker had neither made the allegation nor provided evidence to support it in his testimony. Lula denied receiving money from the banker.

Flávio Bolsonaro is being investigated over what happened to the £20m he was caught on tape asking Vorcaro to provide for a biopic about his father. One suspicion is that the money helped cover living and travel expenses for his brother, the former congressman Eduardo Bolsonaro, during his self-imposed exile in the US, where he lobbied the Trump administration against Lula. The senator said all the money went towards sponsoring the film, which still has no release date.

Besides Flávio Bolsonaro’s lead, the far right’s sweeping victory ensured an even more conservative congress in 2027, including the senate majority needed to impeach supreme court justices – a demand among his father’s supporters since the former president was convicted.

“With the scandal, the justices handed Bolsonarismo something it still didn’t have: an impeachable offence that could justify their removal,” said Mendes. “A court that ignores conflicts of interest and immerses itself in relationships with political and economic power is a court that has already become corrupted.”

Even if no justice is impeached, should Bolsonaro win, he could be entitled to fill four seats as justices retire during his term, leaving the court with at least six of its 11 members appointed by either him or his father.

“We can expect a court completely servile to the government,” Mendes said, adding that the separation of powers would be at risk and that the justices themselves bear part of the responsibility.

“The court ended up doing to itself the work an autocratic project needs – and brought itself down”.