Thu Feb 27, 2014 9:18pm EST

(Reuters) – Brazil’s Supreme Court overturned racketeering convictions against former leaders of the ruling Workers Party on Thursday, reducing their prison sentences in the country’s biggest political corruption case.

The 6-5 ruling does not affect other convictions for corruption and money-laundering in the congressional vote-buying scandal that almost toppled party leader Luiz Inacio Lula da Silva from the presidency in 2005.

“This is a sad day for this Supreme Court,” said a disheartened Chief Justice Joaquim Barbosa, who became a household name in Brazil for his pursuit of convictions in the so-called mensalao, or big monthly payments, case.

The 2012 trial was seen as a landmark in a country with a long history of corruption where political leaders had never gone to jail for bribery or embezzlement of public funds. Twenty-five people were convicted.

Lula was never implicated in the scheme that involved monthly payments to lawmakers in exchange for support in Congress for his minority first government.

The court ruling that the racketeering convictions lacked proof was welcomed as “just” by the Worker’s Party.

The decision will benefit Jose Dirceu, a party founder and Lula’s former chief of staff, whose 7-year-and-11-month prison sentence will be reduced, allowing him to serve under house arrest next year and be paroled in 2016.

Jose Genoino, a former party president, and Delubio Soares, a former party treasurer, also will serve less time.

The shorter sentences will not diminish the trial’s historic significance as a step toward ending impunity in Brazil, said Senator Aloysio Nunes, a leader of the opposition PSDB party.

“No matter how long they serve, the fact is that for the first time important people were put on trial for corruption and convicted,” Nunes told reporters.

Lula’s hand-picked successor, President Dilma Rousseff, has distanced herself from the scandal and gained a reputation for not tolerating corruption in her administration.

Rousseff weathered massive protests in June by Brazilians disgusted with corruption and bad public services. She is favored to win re-election in October and keep the Workers’ Party in power for another four years.