RIO DE JANEIRO >> An appeals court in southern Brazil today upheld a corruption conviction against the former president, Luiz Inácio Lula da Silva, dealing a significant blow to his quest to win a third term in office.

The ruling was a victory for prosecutors in what may be the highest-stakes case in the yearslong showdown between Brazil’s judiciary and the political elite. Prosecutors have portrayed da Silva, who has also been charged in six other corruption cases, as a linchpin of Brazil’s endemically corrupt political system.

The unanimous decision by a three-judge panel narrows the paths da Silva could have pursued to get the conviction overturned and adds fresh uncertainty to the deeply polarized race to replace President Michel Temer next year. The judges sentenced da Silva to 12 years in prison, lengthening the term imposed by the trial judge.

Under Brazil’s “Clean Slate” law, which Congress passed in 2010 in response to public outcry about rampant political corruption, the ruling makes da Silva ineligible to run in the presidential election scheduled for October. But he is widely expected to continue fighting for the right to appear on the ballot.

His prosecution and potential disbarment from running for office have raised doubts about the legitimacy of the election because da Silva has carved out a significant and sustained lead in the polls.

Da Silva, 72, was defiant on Tuesday as he headlined a rally in Pôrto Alegre, where the Fourth Regional Federal Court of Appeals, which upheld the conviction, is based.

“Only one thing is going to remove me from the streets of this country and that will be the day that I die,” he said. “Until then, I will be fighting for a fairer society.”

An election victory by da Silva would be a dramatic return to power for him and his leftist Workers’ Party, two years after the impeachment of his protégé, former president Dilma Rousseff. Her removal elevated Temer of the center-right Brazilian Democratic Movement party to the presidency.

Federal Judge Sergio Moro, the most prominent figure in the judiciary’s crackdown on political corruption, convicted da Silva in July on corruption and money laundering charges for accepting bribes from OAS, a major construction company, in the form of a seaside apartment that was being renovated to his liking. The judge sentenced da Silva to nearly a decade in prison, but ruled that he should remain free pending appeals.

Da Silva, who is widely known as Lula, has called the conviction a miscarriage of justice orchestrated by underhanded political actors within the judiciary.

“It’s clear that this is not a legal trial based on laws, but a political trial aimed at convicting Lula at any cost, even if he’s an innocent man,” Cristiano Zanin Martins, da Silva’s lawyer, said earlier this week. “These actions are clearly politically motivated in order to remove him from the political arena.”

Da Silva’s supporters claim that he never lived in, or took ownership of, the renovated apartment at the heart of the case. They have cast doubt on the reliability of witnesses who implicated him, asserting that the witnesses testified in return for leniency in their own corruption cases.

They have also noted that politicians accused of more egregious wrongdoing — including Temer, who was recorded appearing to condone the payment of a bribe — have so far dodged accountability.

The three-judge panel that considered da Silva’s appeal included two jurists appointed by the former president’s political ally, Rousseff: João Pedro Gebran Neto and Leandro Paulsen. The third judge, Victor Laus, was appointed by the former president Fernando Henrique Cardoso.

“There is evidence that established beyond a reasonable doubt that the ex-president was one of the leaders, if not the leader, of a vast corruption scheme,” Judge Gebran Neto said during the hearing. “At a minimum, the evidence shows that he was aware of it and supported it.”

Anamara Osório Silva, the president of the National Association of Prosecutors, decried what he said were efforts to impugn the integrity of the judiciary.

“This is not a politicized prosecution, it is the prosecution of a politician accused of committing grave crimes,” Osório said on Wednesday. “No one can be beyond the reach of an independent judicial branch. No one will intimidate or weaken the resolve of our prosecutors.”

Da Silva could go before Brazil’s Superior Electoral Court and the Supreme Federal Court to argue that disqualifying him from running in the election would subvert democracy. Political analysts and legal experts in Brazil say that ultimately, the Supreme Court is likely to resolve the matter.

Da Silva is not expected to be jailed while appeals are pending. His backers have warned that imprisoning the former president, who governed Brazil from 2003 to 2010, would trigger a severe response from Workers’ Party loyalists, known by its initials in Portuguese, PT.

“The reaction from the PT would be to grab him from prison,” Sen. Gleisi Hoffmann, the president of the party, said in an interview last year. “There’s a culture in Brazil on this issue of arresting a president, an ex-president, that I think that even the armed forces would not allow it.”

Senior politicians and Brazilian scholars have warned in recent days that barring da Silva from running would further undermine faith in the country’s young democracy, which has been rocked in recent years by the impeachment of Rousseff and by the sprawling corruption inquiry known as Lava Jato, or Car Wash, that began in 2014.

Temer said last week that he hoped da Silva would be allowed to run.

“I think if Lula participates, it would be a democratic thing, the people will say whether they want him or not,” the president said in an interview with the Folha de São Paulo newspaper that was published on Saturday. “If he is defeated politically it is better than being defeated” in the courts, which would make him seem like a victim, the president added.

Da Silva is expected to travel to Ethiopia later this week for a meeting with African leaders.

During a meeting with journalists last week, the former president, who rose to prominence as a union activist during Brazil’s 1964-85 military dictatorship, characterized the case against him as merely the latest form of persecution he has faced.

“I think that those who accuse me today are more worried than I because I have the peace of mind of those who are innocent,” da Silva said. “And they must be feeling the guilt that comes from lying.”

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