When Mr. Barbosa was a teenager, he was employed as a

     BRASÍLIA — Brazil’s highest court has long viewed itself as a bastion of manners and formality. Justices call one another “Your Excellency,” dress in billowing robes and wrap each utterance in grandiloquence, as if little had changed from the era when marquises and dukes held sway from their vast plantations.

     In one televised feud, Mr. Barbosa questioned another justice about whether he would even be on the court had he not been appointed by his cousin, a former president impeached in 1992. With another justice, Mr. Barbosa rebuked him over what the chief justice considered his condescending tone, telling him he was not his “capanga,” a term describing a hired thug. 

      In one of his most scathing comments, Mr. Barbosa, the high court’s first and only black justice, took on the entire legal system of Brazil — where it is still remarkably rare for politicians to ever spend time in prison, even after being convicted of crimes — contending that the mentality of judges was “conservative, pro-status-quo and pro-impunity.”

     “I have a temperament that doesn’t adapt well to politics,” Mr. Barbosa, 58, said in a recent interview in his quarters here in the Supreme Federal Tribunal, a modernist landmark designed by the architect Oscar Niemeyer. “It’s because I speak my mind so much.” 

     His acknowledged lack of tact notwithstanding, he is the driving force behind a series of socially liberal and establishment-shaking rulings, turning Brazil’s highest court — and him in particular — into a newfound political power and the subject of popular fascination. 

   The court’s recent rulings include a unanimous decision upholding the University of Brasília’s admissions policies aimed at increasing the number of black and indigenous students, opening the way for one of the Western Hemisphere’s most sweeping affirmative action laws for higher education. 

     In another move, Mr. Barbosa used his sway as chief justice and president of the panel overseeing Brazil’s judiciary to effectively legalize same-sex marriage across the country. And in an anticorruption crusade, he is overseeing the precedent-setting trial of senior political figures in the governing Workers Party for their roles in a vast vote-buying scheme.

   Ascending to Brazil’s high court, much less pushing the institution to assert its independence, long seemed out of reach for Mr. Barbosa, the eldest of eight children raised in Paracatu, an impoverished city in Minas Gerais State, where his father worked as a bricklayer.  

    But his prominence — not just on the court, but in the streets as well — is so well established that masks with his face were sold for Carnival, amateur musicians have composed songs about his handling of the corruption trial and posted them on YouTube, and demonstrators during the huge street protests that shook the nation this year told pollsters that Mr. Barbosa was one of their top choices for president in next year’s elections.

     While the protests have subsided since their height in June, the political tumult they set off persists. The race for president, once considered a shoo-in for the incumbent, Dilma Rousseff, is now up in the air, with Mr. Barbosa — who is now so much in the public eye that gossip columnists are following his romance with a woman in her 20s — repeatedly saying he will not run. “I’m not a candidate for anything,” he says. 

     But the same public glare that has turned him into a celebrity has singed him as well. While he has won widespread admiration for his guidance of the high court, Mr. Barbosa, like almost every other prominent political figure in Brazil, has recently come under scrutiny. And for someone accustomed to criticizing the so-called supersalaries awarded to some members of Brazil’s legal system, the revelations have put Mr. Barbosa on the defensive. 

     One report in the Brazilian news media described how he received about $180,000 in payments for untaken leaves of absence during his 19 years as a public prosecutor. (Such payments are common in some areas of Brazil’s large public bureaucracy.) Another noted that he bought an apartment in Miami through a limited liability company, suggesting an effort to pay less taxes on the property. In statements, Mr. Barbosa contends that he has done nothing wrong. 

     In a country where a majority of people now define themselves as black or of mixed race — but where blacks remain remarkably rare in the highest echelons of political institutions and corporations — Mr. Barbosa’s trajectory and abrupt manner have elicited both widespread admiration and a fair amount of resistance. 

     As a teenager, Mr. Barbosa moved to the capital, Brasília, finding work as a janitor in a courtroom. Against the odds, he got into the University of Brasília, the only black student in its law program at the time. Wanting to see the world, he later won admission into Brazil’s diplomatic service, which promptly sent him to Helsinki, the Finnish capital on the shore of the Baltic Sea. 

     Sensing that he would not advance much in the diplomatic service, which he has called “one of the most discriminatory institutions of Brazil,” Mr. Barbosa opted for a career as a prosecutor. He alternated between legal investigations in Brazil and studies abroad, gaining fluency in English, French and German, and earning a doctorate in law at Pantheon-Assas University in Paris. 

   Fascinated by the legal systems of other countries, Mr. Barbosa wrote a book on affirmative action in the United States. He still voices his admiration for figures like Thurgood Marshall, the first black Supreme Court justice in the United States, and William J. Brennan Jr., who for years embodied the court’s liberal vision, clearly drawing inspiration from them as he pushed Brazil’s high court toward socially liberal rulings.

    Still, no decision has thrust Mr. Barbosa into Brazil’s public imagination as much as his handling of the trial of political operatives, legislators and bankers found guilty in a labyrinthine corruption scandal called the mensalão, or big monthly allowance, after the regular payments made to lawmakers in exchange for their votes. 

    Last November, at Mr. Barbosa’s urging, the high court sentenced some of the most powerful figures in the governing Workers Party to years in prison for their crimes in the scheme, including bribery and unlawful conspiracy, jolting a political system in which impunity for politicians has been the norm.  

     Now the mensalão trial is entering what could be its final phases, and Mr. Barbosa has at times been visibly exasperated that defendants who have already been found guilty and sentenced have managed to avoid hard jail time. He has clashed with other justices over their consideration of a rare legal procedure in which appeals over close votes at the high court are examined. 

     Losing his patience with one prominent justice, Ricardo Lewandowski, who tried to absolve some defendants of certain crimes, Mr. Barbosa publicly accused him this month of “chicanery” by using legalese to prop up certain positions. An outcry ensued among some who could not stomach Mr. Barbosa’s talking to a fellow justice like that. “Who does Justice Joaquim Barbosa think he is?” asked Ricardo Noblat, a columnist for the newspaper O Globo, questioning whether Mr. Barbosa was qualified to preside over the court. “What powers does he think he has just because he’s sitting in the chair of the chief justice of the Supreme Federal Tribunal?” 

      Mr. Barbosa did not apologize. In the interview, he said some tension was necessary for the court to function properly. “It was always like this,” he said, contending that arguments are now just easier to see because the court’s proceedings are televised. 

     Linking the court’s work to the recent wave of protests, he explained that he strongly disagreed with the violence of some demonstrators, but he also said he believed that the street movements were “a sign of democracy’s exuberance.” 

     “People don’t want to passively stand by and observe these arrangements of the elite, which were always the Brazilian tradition,” he said.