Silvio Berlusconi faces expulsion from the Italian parliament through a momentous vote prompted by his criminal conviction for tax fraud.
The culmination of months of political wrangling, the vote on Wednesday opens an uncertain new phase for one of Italy's most divisive figures, who has dominated politics for two decades.
The 77-year-old media billionaire and former prime minister had asked fellow senators to delay the vote, claiming to have new evidence warranting a judicial review of his conviction, but it is expected to go ahead at 1800 GMT.
"Berlusconi is still extremely powerful, although that power is declining," said James Walston, a professor at the American University in Rome.
"He still has enormous resources, he still has his media, he still has lots of very diehard supporters inside and outside parliament."
Thousands of those supporters were expected to mass outside Berlusconi's luxury home in Rome on Wednesday afternoon, while a smaller group of anti-Berlusconi activists were planning to gather outside the Senate.
The vote comes at an economically crucial time for Italy, which is struggling to end its longest post-war recession, and just as parliament debates next year's budget.
Prime Minister Enrico Letta has called for a "non-chaotic situation in Italy," saying a division within Berlusconi's ranks "will help stability."
A group of dissidents led by Berlusconi's former protege, Deputy Prime Minister Angelino Alfano, broke away earlier this month to form their own grouping.
Alfano and his supporters have said they will vote against Berlusconi's expulsion but will stay in Letta's left-right coalition even if the ejection goes ahead.
This would prevent a government collapse, even though Berlusconi's Forza Italia party on Tuesday announced it was pulling out of the coalition after just six months of uneasy cohabitation with its leftist rivals.
But the coalition would have a much narrower Senate majority of around 10 seats, and could be vulnerable to sniping from Berlusconi even outside parliament.
Berlusconi has said a vote to expel him from parliament would be an "indelible stain" on Italian democracy.
"[It] would shame you in front of your children, your voters and all Italians," he said in an open letter to senators, maintaining the vote was "not about me, but about democracy."
Even some of Berlusconi's most hardline opponents have voiced doubts about the expulsion vote and the idea of beating the 77-year-old through the courts and parliament instead of at the ballot box.
Ejection from the Senate would also remove Berlusconi's parliamentary immunity, which offers a series of safeguards against the arrest of lawmakers, and observers say he is concerned about a possible arrest - although his lawyers dismiss the prospect as "completely unrealistic."
Berlusconi is currently appealing convictions for having sex with an underage prostitute, abusing the powers of the prime minister's office and leaking a confidential police wiretap to damage a political rival.
He also faces trial for bribing a leftist senator to join his party's ranks and could come under investigation for paying off young women who attended his raunchy parties to give favourable testimony.