Rosy Bindi, president of the opposition Democratic Party and deputy speaker of the lower house of parliament, said in an interview sex scandals were likely to cost Berlusconi power.
"Italian women are paying the highest price for this scandal, with an image being spread of women whose value is reduced to their bodies and who become tradeable goods at the disposal of the sultan, the emperor," she told Reuters.
"Berlusconi has based an important part of his electoral success on women ... but undoubtedly his approval ratings are now falling ... women will represent a key element in putting an end to Berlusconi's honeymoon in our country," she said in her office in the lower house on Wednesday evening.
Milan prosecutors have alleged that Berlusconi paid for sex with prostitutes who attended parties at his villa, including a teenaged nightclub dancer who goes by the stage name of "Ruby the Heart-Stealer".
The billionaire media mogul denies any wrongdoing and says he has never paid for sex. He says politically motivated leftist magistrates are bent on destroying him and he has refused to face questioning.
Berlusconi has survived previous scandals about his private life. Middle-aged and elderly women have been key sources of electoral support for the prime minister.
But Bindi, who has called for Berlusconi to resign, said the most recent scandal would likely be seen as a step too far.
"In the last weeks there has been a jump to a new level: before it was our head of government who was ridiculed, but now we are all at risk of being ridiculed," she said.
Newspapers have run transcripts of phone taps by magistrates which paint a picture of wild parties attended by Berlusconi, other older men and young starlets and models who received money and favours like free rent.
Berlusconi has frequently joked about Bindi, once calling the bespectacled, grey-haired 59-year-old "more beautiful than intelligent".
That stirred a backlash from thousands of Italian women, and her quick response -- "I'm not a woman at your disposal""-- became a rallying cry printed on T-shirts and placards.
Bindi said then there was a "new feminism""taking root in Italy, where scantily clad women are a common sight on TV, especially on channels owned by Berlusconi.
"When Berlusconi starts to offend me or others, I rely on my inner strength, and in that case I told him I'm not at his disposal like a woman he can buy," she told Reuters.
Italy's next elections are not scheduled until 2013 but most commentators say either Berlusconi's sex scandals or the fracturing of the centre right will lead to the collapse of his government and early national polls, perhaps this year.
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