SÃO PAULO, Brazil >> Just as former President Luiz Inácio Lula da Silva, the front-runner in Brazil’s presidential election, was surrendering to start serving a 12-year sentence for corruption this month, a former Supreme Court justice quietly made his debut on the political stage, quite possibly upending the contest.

Joaquim Barbosa, who made history in 2003 when he became the country’s first black Supreme Court justice, registered with the center-left Brazilian Socialist Party on April 6, one day before the deadline for potential presidential candidates to join a party.

While he has yet to formally begin a campaign, party leaders have spent the past few weeks building a strategy that draws on Barbosa’s remarkable biography. Having overcome poverty and discrimination to reach the pinnacle of the legal profession, Barbosa became a crusading figure in the fight against corruption, which is the top concern among Brazilian voters.

“His campaign won’t be based on polarization,” Carlos Siqueira, the president of the party, said in an interview. “It will be about his clean, honest name and about a black man who comes from humble origins and made it to the Supreme Court and now could reach the presidency.”

The first poll conducted by Datafolha in the wake of da Silva’s imprisonment, which was released April 15, puts Barbosa in third place, with his level of support at 10 percent, in a matchup that excludes the former president. The poll had a margin of sampling error of 2 percent and included more than 4,100 interviews.

Barbosa’s standing in the poll is remarkable considering that he has stayed out of public view and still has not said whether he will, in fact, run.

“For someone who doesn’t frequent public spaces, doesn’t give interviews, leads a quiet life, it’s pretty good,” Barbosa said earlier this week when reporters asked about his poll numbers as he walked into a meeting with party leaders in Brasília, the capital.

But he cautioned that his candidacy was not a done deal, citing unspecified “personal difficulties.”

Eurasia, a consultancy that closely tracks Brazilian politics, recently called Barbosa “the real wild card of this election.”

With six months to go before Brazilians cast their ballots in the most unpredictable and splintered presidential election since the return of democracy in the mid-1980s, the onetime front-runner, da Silva, sits behind bars with no anointed successor for his left-wing base to rally around.

The incumbent, President Michel Temer, is among the old-guard leaders who have become widely despised by the electorate amid mounting revelations of systemic corruption by the political chieftains who have run Brasília for decades.

That may set the stage for a remarkable showdown.

Excluding da Silva, the leader in the polls is Jair Bolsonaro, a congressman and ultraconservative former army captain who was recently charged with inciting racism and discrimination against blacks.

Trailing him are Marina Silva, a former environment minister, and Barbosa, who are among the few black people who managed to muscle their way into the top echelons of power in Brazil.

Slightly more than half of Brazilians describe themselves as black or mixed race.

While Silva has competed in the past two presidential elections, Barbosa is a fresh face on the political scene, which may prove an asset at a time when voters are clamoring for a break with the past.

The eldest of eight children, Barbosa was raised in the poor city of Paracatu in Minas Gerais state, where his father worked as a bricklayer. As a teenager, he worked as a janitor in a courtroom in Brasília. He was the only black student in his law school class at the University of Brasília.

Barbosa, 63, began his government career with a short stint as a diplomat, but left the foreign service after concluding that he would not advance much in a bureaucracy he found hostile to blacks. He studied abroad, learned English, French and German, and worked as a federal prosecutor before becoming a judge.

Barbosa was appointed to the Supreme Court by da Silva in 2003, and he led the charge against politicians implicated in a kickbacks scandal known as mensalão, a reference to monthly payments to lawmakers in exchange for votes. The investigation landed several stalwarts of da Silva’s Workers’ Party in prison.

The election, which will take place in October, is the first since a subsequent corruption investigation known as Lava Jato, or Car Wash, which tarred much of the political elite.

Just this week, Aécio Neves, the presidential candidate who was narrowly defeated in 2014, was ordered to stand trial before the Supreme Court after he was accused of accepting a bribe and obstructing justice. Temer is accused in two corruption cases, but no trial as been set.

During his tenure on the court, from which he retired in 2014, Barbosa became famous for his blunt style and the barbs he delivered from the bench.

Yet it remains a mystery how he would act on a campaign trail. Also unclear is how many traditional supporters of da Silva, who is widely expected to designate an heir in coming months, would gravitate toward Barbosa.

None of the leading candidates have the support of a political party with a strong nationwide presence. Ana Lúcia, a public affairs specialist in Rio de Janeiro, said that could be Barbosa’s Achilles’ heel.

“He’s a person of integrity who fights corruption,” she said. “But on the other hand, he’s new to politics, he doesn’t have party connections, and because of that he couldn’t make the necessary changes.”

Barbosa may be in a strong position to attract voters who once supported da Silva.

Miguel Oliveira, a 47-year-old maintenance worker whose family is from the poor northeast of Brazil, said he had always voted for “Lula” as da Silva is universally known.

“But I’d definitely take a look at Joaquim Barbosa,” Oliveira said. “He at least knows what it means to be poor in a country where politicians are stealing all of the money.”