Calls for Rousseff’s impeachment have centred on allegations, unrelated to Petrobras, that she broke budget rules to boost spending as she campaigned for re-election in 2014. A 65-member impeachment committee in the lower house of Congress will now study if there are grounds to try her in the Senate.

Rousseff and Lula have both denied any wrongdoing.

Deliberations could drag on for months on the impeachment committee. Seats were spread among all parties in the lower house of Congress, but the key roles of committee president and rapporteur were given to close allies of Speaker Eduardo Cunha, a sworn political enemy of the president.

Rousseff appointed her mentor, who remains one of Brazil’s most influential politicians six years after leaving office, in an effort to fight impeachment and win back working-class supporters amid the worst economic recession in decades.

The corruption probe, however, has weakened Lula’s sway in Congress and there are growing signs that Rousseff’s main coalition partner is ready to abandon the unpopular government.

“UNDERGROUND WAR”

As Rousseff swore Lula into office, she strongly criticised the release on Wednesday of a taped telephone conversation between them that was made public by Sergio Moro, the crusading federal judge overseeing the Petrobras investigation.

Moro, the public face of the biggest graft probe in Brazilian history, said the tape showed they had discussed influencing prosecutors and courts to protect Lula, who leaves Moro’s jurisdiction by joining the government.

“Convulsing Brazilian society with lies, with reprehensible practices violates constitutional rights and as well as the rights of citizens,” said Rousseff, who called the recording illegal and anti-democratic.

Judiciary associations and prosecutors working with Moro sprang to his defence, repudiating what they called efforts to intimidate him and his team of investigators.

“Those telephone conversations provide evidence of obstructing the investigation, part of a treacherous underground war waged in the shadows, far from the courts,” wrote 13 federal prosecutors on Moro’s task force in a public statement.

Moro said in a court filing released on Wednesday that the taped telephone conversations did not provide proof that Lula and Rousseff were interfering with his investigation.