SAO PAULO (Reuters) - Brazil’s Supreme Court ruled on Thursday that corruption cases involving illegal campaign donations should be handled by electoral courts, a move that some prosecutors warned could hurt their groundbreaking crusade against graft.

Six justices on the nation’s top court cast votes in favor of sending to electoral courts all cases involving politicians who use money gained through corruption or money laundering for campaigns, rather than having them heard by federal criminal courts. Five justices opposed the decision.

Prominent federal prosecutors warned that could unwind the five years of work carried out during the landmark “Car Wash” corruption investigation, which has sent scores of powerful politicians and businessmen to jail in the world’s biggest graft probe of its kind.

Many of the Car Wash convictions - including that of imprisoned former president Luiz Inacio Lula da Silva - involved illegal campaign donations as an aspect of the case. Prosecutors argue that could open the door for Lula and other convicts to ask the top court to annul their sentences and send their cases to dedicated electoral courts.

“For those who have committed these crimes, this will really be a prize,” Deltan Dallagnol, the lead prosecutor overseeing the Car Wash investigation, said in a written statement.

“Defense lawyers for these people will argue their crime was related to illegal campaign financing ... to move the case to electoral courts and lessen the chances of a successful investigation and serious punishment.”

Senator Olimpio Gomes, a strong ally of far-right President Jair Bolsonaro, who was elected on an anti-corruption platform, tweeted just after the decision that he had already introduced a bill to reverse it. The bill would “guarantee the fight against organized crime, corruption and impunity,” he said.

Others backed the majority in the Supreme Court ruling, saying their argument adhered to the Brazilian constitution.

‘NOT PREPARED’

Brazil’s unique electoral justice system is made up of a rotating cast of state and federal judges that handle an array of campaign disputes surrounding local, state and national elections every two years.

Critics argue that makes them less prepared than federal criminal courts to address the country’s political graft schemes uncovered in recent years.

Justice Minister Sergio Moro, who before taking his government post was the federal judge who spearheaded the Car Wash trials and sent Lula to jail, said on Wednesday that the Supreme Court sending the cases to an electoral court would be a blow for the country’s push against impunity.

“Electoral courts ... are not prepared to judge more complex crimes, such as money laundering and corruption,” Moro said.

Carlos Melo, a political scientist with Sao Paulo business school Insper, agreed with Moro’s assessment of the electoral courts. However, he said the constitution clearly states that crimes connected to illegal campaign financing must be judged by electoral courts.

“Will this hurt the Car Wash investigation? Sure, it is a blow. But the prosecutors have ridden a wave of wins, jailed powerful figures like Lula, and there is the sense they want to have everything go their way,” Melo said.

If Brazilians want to make prosecutors’ work easier, they will have to find other ways, he said.

“It is now up to Brazilian society to pressure lawmakers and demand changes to the law,” he said. “Or they can demand that the electoral courts are strengthened so they can deal with these types of cases.”