The ruling that overturned ex-president Luiz Inacio Lula da Silva's corruption convictions upended Brazilian politics and set up a potential election showdown between the tarnished left-wing icon and his nemesis, far-right President Jair Bolsonaro.

Monday's decision by Supreme Court Justice Edson Fachin overturned all convictions against the former president (2003-2010) stemming from a probe into a massive corruption scheme centered on Brazilian state oil company Petrobras.

The ruling was procedural, but it reinstates Lula's right to run for office while the cases against him play out in a different court.

The news dropped like a bomb just as Brazil gears up for presidential elections in October next year, when the country -- deeply divided over Bolsonaro's combative, in-your-face reign -- will decide whether to keep the man dubbed the "Tropical Trump" for another four years.

Now, the incumbent faces the prospect of a heavyweight adversary on the left, rewriting a race that had looked to be shaping up as a battle between Bolsonaro and a raft of candidates vying for the center.

"This is going to polarize the narrative," said political analyst Murillo de Aragao, of consulting firm Arko Advice.

"Now Bolsonaro can radicalize his message and Lula can say he was persecuted."

Lula, who at 75 remains as charismatic as he is controversial, has always claimed he was innocent.

The former steelworker says the charges against him were fabricated to sideline him from the 2018 presidential race, in which he was the frontrunner.

In April that year, he was jailed for taking a bribe from a construction company in return for juicy Petrobras contracts.

After more than a year and a half behind bars, a Supreme Court ruling freed him pending appeal.

By then, Bolsonaro had surged to the presidency, riding the outrage with the giant corruption scandal engulfing Lula, his Workers' Party (PT), and much of Brazil's political and business establishment.

But just over two years into his term, Bolsonaro looks vulnerable.

His defiance of expert advice on fighting the coronavirus pandemic has proved a risky bet as Covid-19 has devastated Brazil, claiming more than 266,000 lives -- the second-highest death toll worldwide, after the United States.

The coalition of forces that brought the former army captain to power has meanwhile frayed.

Bolsonaro has clashed with both the anti-corruption faction -- falling out with popular ex-justice minister Sergio Moro, the former judge who jailed Lula -- and the business sector, alarmed by his increasing turn to big-spending economic populism.

Bolsonaro, 65, dismissed Lula's return, saying he was unconcerned.

"The (Lula) government's robbery is plain for all to see," he said.

"I don't think the Brazilian people even want a (PT) candidate in 2022, much less him."

But opinion polls suggest otherwise.

The latest, released Sunday by polling firm Ipec, gave Lula the most potential votes in the 2022 election, with 50 percent -- the only politician to outperform Bolsonaro (38 percent).

Newspaper Estado de Sao Paulo called the latest twist a "macabre feat" for the president, in a mocking editorial.

"Bolsonaro is managing to do what seemed impossible: shirk his responsibilities to the point that he has paved the way for Lula to make a political come-back."

But there are also whispers in Brasilia that Bolsonaro is happy to have his old enemy back, and with him, a return to a familiar script.

"It is no secret the presidential palace has been rooting for Lula's eligibility," consulting firm Eurasia Group said in a note.

"It creates a foil for the president and increases his odds of a second-round victory in 2022 if he faces a candidate on the left rather than the political center."

Lula would meanwhile face the challenge of uniting a left wing that has badly unraveled since he went to jail.

The Workers' Party is now a shadow of its former self, with upstart rivals challenging its dominance on the left.

If Lula is a candidate, "don't count me in to take part in this circus," tweeted prominent center-left politician Ciro Gomes.

The fiery veteran finished third in the 2018 presidential race, behind Bolsonaro and Lula's stand-in, Fernando Haddad.

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