(Adds comments from mayor and minister's lawyer in paragraphs 4, 5)
SAO PAULO, Feb 24 (Reuters) - Brazil's Supreme Court on Wednesday authorized formal investigations into potential corruption involving President Dilma Rousseff's former chief of staff, as well as the mayor of the country's largest city and an opposition senator.
Supreme Court Justice Celso de Mello said federal prosecutors could investigate Rousseff's former chief of staff, Aloizio Mercadante, who now serves as education minister, Sao Paulo Mayor Fernando Haddad and Senator Aloysio Nunes of the opposition PSDB party.
Prosecutors said all three were named in plea bargain deals with states' witnesses in a corruption and political kickback scheme at state-run oil firm Petroleo Brasileiro SA, but that the possible crimes raised by the witnesses were not necessarily related to the Petrobras investigation.
Haddad said in a statement that he had nothing to hide and that his campaign accounts have already been approved by the electoral court of the state of Sao Paulo.
Mercadante's lawyer, Pierpaolo Cruz Bottini, told Reuters that the judge's decision does not open a new probe but only extends an ongoing investigation.
Senator Nunes also said in a statement that the decision only includes the witness testimony to the ongoing investigation that should move quickly to clear his name.
Around 50 politicians, mostly from Rousseff's coalition, are under investigation for possibly taking bribes in Brazil's largest-ever corruption investigation.
Some politicians have been cleared of wrongdoing after they were named in plea bargain testimony.
Dozens of engineering executives are on trial for allegedly forming a cartel to fix prices on contracts with Petrobras, as the oil firm is known, and using the excess funds to bribe politicians and Petrobras executives. (Reporting by Eduardo Simoes; Writing by Caroline Stauffer; Editing by Leslie Adler and David Gregorio)