(This version of the Oct. 25 story corrects story to change name to Emilio Lozoya in paragraph three)

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MEXICO CITY (Reuters) - A prosecutor who led an investigation into a graft scandal involving the campaign of Mexican President Enrique Pena Nieto said on Wednesday he was fired without justification, as the Senate prepares to vote on whether to reinstate him.

Mexico’s acting attorney general fired Santiago Nieto, the attorney general for election-related crimes, last week on grounds that he broke a code of conduct.

His summary dismissal came just days after an explosive interview with newspaper Reforma in which Nieto accused Emilio Lozoya, the former boss of state oil firm Pemex and a senior member of Pena Nieto’s 2012 campaign team, of writing to him to ask that he be declared innocent of funneling Odebrecht cash into the president’s campaign.

Brazilian construction firm Odebrecht is at the heart of a Brazilian bribery and kickback probe, known as Lava Jato or Car Wash, that has reverberated across Latin America.

Javier Coello Trejo, Lozoya’s lawyer, said on Wednesday he had filed two criminal complaints against Nieto, alleging the former prosecutor had violated laws governing criminal proceedings, broadcaster Televisa reported.

Nieto’s firing angered opposition politicians and has even threatened to delay discussions among lawmakers about next year’s budget.

In Wednesday’s television interview, Nieto denied breaking any rules, or revealing any sensitive information.

“I didn’t violate any code of ethics,” said Nieto, who was leading the probe targeting both Lozoya and the president’s campaign.

The Senate is due to vote in coming days on whether to reinstate Nieto, but it is still not clear when the vote will be and if it will be public.

In a sign of how tempers have frayed over the issue, opposition lawmakers have promised to delay voting on the 2018 budget until Nieto’s case is resolved.

The dispute is complicating efforts to shake off graft allegations that could hurt the ruling Institutional Revolutionary Party (PRI) ahead of next year’s presidential election.

Nieto could not be reached immediately for comment on Wednesday. Both the president and Lozoya, his close ally, have previously denied involvement in any wrongdoing related to the 2012 campaign.

Mindful that corruption has become a key issue in the 2018 election, the PRI has overseen the arrest of various former state governors. Lozoya poses a more difficult challenge, given his proximity to the president.

Opposition lawmakers have also criticized a decision by the Senate’s governance body to hold the Nieto vote in private, vowing to appeal against the ruling.