Curitiba - Brazil's once hugely popular president Luis Inacio Lula da Silva underwent a new grilling on Wednesday by the country's chief anti-corruption judge, Sergio Moro, in a case that could decide whether Lula can return to power.Lula, a frontrunner in the October 2018 presidential election despite multiple corruption charges, was greeted by several hundred leftist supporters on arrival at the courthouse in the southern city of Curitiba.Demonstrators wearing red shirts crowded around the 71-year-old, shouting encouragement. About 1 500 police officers kept a close watch.Inside, Moro questioned Lula about alleged bribe taking from the scandal-plagued Odebrecht construction giant.Moro is a key figure in the mammoth "Car Wash" anti-graft drive which has uncovered systemic bribery by corporations of Brazil's political class, as well as mass embezzlement from state oil company Petrobras.Lula, arguably Moro's biggest target, is already a defendant in four other corruption trials.And in July, Moro sentenced Lula to 9.5 years in prison after being convicted in a sixth trial of receiving a seaside apartment from the OAS construction company in return for help obtaining lucrative contracts with Petrobras.The founder of Brazil's leftist Workers' Party, Lula is free pending appeal of that sentence and hopes to string out or defeat the other cases so that he can run for a third term next year.But problems are mounting for the man whose 2003-2010 presidency made him Latin America's new leftist giant.Corruption 'blood pact' Although Lula tops the polls, that is partly because campaigning hasn't started and name recognition gives him a head start. Also, his negative ratings are far higher than for other candidates.A recent bus tour of his electoral heartland in the north-east of Brazil met with a lukewarm reception, drawing core supporters but not the kind of massive crowds that might have made judges afraid of tough action.Lula says the corruption charges are concocted to prevent his return.However, he has been hurt by testimony from his former economy minister and right hand man, Antonio Palocci, who told Moro last week that "the facts related in (the latest case involving Lula) are true."Lula is accused of having Odebrecht buy the land for his Sao Paulo institute and providing an apartment in nearby Sao Bernardo do Campo.Palocci said a "blood pact" was reached between the Workers' Party and Odebrecht in 2010 when Lula was handing over to his successor Dilma Rousseff, with the company giving the party the equivalent at that time of $171 million.Lula says that Palocci, who has been sentenced to 12 years prison, is testifying falsely under pressure.After testifying on Wednesday, Lula was expected to address supporters in Curitiba. The crowd is expected to be smaller than the approximately 7 000 that turned out for Lula when he was questioned by Moro in a previous case in May.Despite Lula's claims to be a victim of a plot by Brazil's right to prevent any repeat of the left's decade in power, "Car Wash" prosecutors have gone after major figures across the political spectrum.President Michel Temer, who took power last year after the impeachment of Lula's handpicked successor Rousseff, is himself currently fighting off multiple corruption probes.