(By Christopher Livesay) Rome, August 19 - Three-time premier Silvio Berlusconi said Monday his leadership of the centre right would not be threatened by his first definitive criminal conviction earlier this month and a ban from office. "I'm resisting! I'm not giving up," Berlusconi told supporters on his Facebook page. "Rest assured that I am not stepping aside, I will remain the head of the center right. I will pursue the interests of Italians and the country until the very end. Go forth with courage". Berlusconi's tax-fraud conviction came with a four-year jail term that, due to an amnesty, was commuted to one year of either community service or house arrest, something the ex-premier and sitting Senator must decide in October. The supreme Cassation Court decision also came with a six-year ban from public office, which must be ratified in the Senate next month before it becomes effective. Given the center left's majority in the Upper House of parliament, the ban is expected to be upheld. A prominent member of Berlusconi's People of Freedom (PdL) party said Monday it would sink the government if the PD votes to eject the ex-premier. Former Senate Speaker Renato Schifani said the ban must be examined by the Constitutional Court before its application is considered. If the PD rules this out it would show a "prejudicial attitude that can only lead to a political rupture," said Schifani. "Co-existence would be impossible", he said. The PdL has been in an unnatural, fragile alliance with old foes the PD for 100 days after a two-month stalemate following inconclusive February elections. Italian President Giorgio Napolitano alluded to the volatility of the cabinet last week when he said he was weighing the possibility of pardoning Berlusconi, while stressing that all convictions must be implemented. Even if a pardon is granted, Berlusconi could still lose his seat in the Senate. Berlusconi loyalist and MP Daniela Santanchè said Monday there was no way that "unelected judges" could "push Berlusconi out as leader of the center right". Former PD minister Cesare Damiano said Monday there would be "no favors or political solutions" to help Berlusconi wiggle out of his first definitive conviction in more than 20 cases, "if not because other cases will be heard shortly". Berlusconi's legal woes - which also include separate but not definitive convictions for sex with an underage prostitute and involvement in the publication of an illegally obtained wiretap that hurt an opponent - have not dented his popularity among the PdL faithful, who are expected to soon rally behind the banner of the retooled Forza Italia party that was the vehicle for his first electoral triumph in 1994.