A scandal has rocked Italy's main opposition party on the eve of a nationwide primary Sunday designed to select a new leader and relaunch the party's sagging fortunes.

The case centers on widespread media reports that a video shows center-left politician Piero Marrazzo in the company of a transsexual in a Rome apartment. Four policemen have been arrested for allegedly attempting to blackmail the 51-year-old Marrazzo, and an investigation is under way.

The video has not been publicly seen and officials have not confirmed its existence. Marrazzo was quoted as saying in Italian newspaper La Repubblica that "the video is fake" and that he is the victim of a smear campaign.

However, bowing to mounting pressure, he said Saturday that he was stepping aside from the high-profile job of governor of the Lazio region, which includes Rome, and leaving his responsibilities to his deputy. Marrazzo said in a statement that the move paved the way for his full-fledged resignation.

"It's a personal case in which weaknesses that have to do with my private sphere have come into play," Marrazzo said in the statement. "The mistakes I have made have in no way interfered with my public activity."

However, he added, the situation has become of such public significance that his staying on was "inappropriate."

The governor, who is married and has three daughters, said he always had his family's best interest in mind.

Marrazzo has not publicly discussed the details of the case, though he said recently that he will talk to the public when the investigation closes.

The case made the front pages of all major Italian newspapers Saturday and dominated TV newscasts.

The Rome prosecutors have not discussed the case publicly. Marrazzo's lawyer could not be reached for comment Saturday.

The scandal exploded just days before center-left supporters are called to the polls to select a new secretary in a bid to relaunch the image of the battered and highly fractious Democratic Party.

The party has been in disarray since losing elections to Premier Silvio Berlusconi's conservatives in April 2008.

Berlusconi has made headlines for months over allegations of inappropriate relations with young women.

The three people vying to become the new leader are Dario Franceschini, who as incumbent secretary has shepherded the party to the primary vote; Pier Luigi Bersani, a former industry minister, and outsider Ignazio Marino.

Bersani is widely seen as the front-runner going into the vote. However, if no candidate wins 50 percent plus one of the vote, a runoff between the top two vote-getters will take place at a party assembly next month.

The three candidates discussed the scandal and in a joint statement Saturday praised Marrazzo's decision to step down as the responsible thing to do.

The four Carabinieri policemen who were arrested in the case were questioned in a Rome jail on Saturday. They are accused of extortion and violation of privacy.