* Senate due to vote on ban over tax fraud conviction

* Berlusconi denounces "coup d'etat"

* Media tycoon can continue to lead centre-right from outside parliament

By James Mackenzie

ROME, Nov 27 (Reuters) - Italian centre-right leader Silvio Berlusconi faces one of the heaviest blows of his 20-year political career on Wednesday when the Senate votes on stripping him of his seat in parliament over a conviction for tax fraud.

The vote will be the culmination of months of political wrangling and is almost certain to lead to Berlusconi's expulsion from the upper house, opening an uncertain new phase for one of Italy's most divisive political figures.

The 77-year-old media billionaire, who has dominated politics for two decades, has already pulled his party out of Prime Minister Enrico Letta's ruling coalition after seven months in government, accusing leftwing opponents of mounting a "coup d'etat" to eliminate him.

The Senate is due to vote at around 7.00 p.m. (1800 GMT) to declare Berlusconi ineligible for parliament after he was convicted of masterminding a complex system of illegally inflated invoices to cut the tax bill for his Mediaset television empire.

The court sentenced him to four years in jail, commuted to a year likely to be spent performing community service, and he was also banned from holding public office for two years, preventing any immediate return to government.

Under a law passed with Berlusconi's support last year, politicians convicted of serious criminal offences are ineligible for parliament, but his expulsion must first be confirmed by a full vote in the Senate.

Both Letta's centre-left Democratic Party (PD) and former comedian Beppe Grillo's anti-establishment 5-Star Movement have declared they will vote against Berlusconi, making it virtually certain that he will be expelled.

His removal will have little immediate impact on Letta's government, which survived a confidence vote on the 2014 budget on Tuesday with the help of a group of some 30 centre-right senators who broke away from Berlusconi's party this month.

But it will heighten the political tensions that have hampered any serious reforms to Italy's stagnant economy, struggling with youth unemployment of more than 40 percent and stuck in a recession that has lasted more than two years.

VICTIM

Once outside, Berlusconi is likely to remain a troublesome opponent for the government, much like Grillo, who does not sit in parliament but who keeps up a steady stream of attacks in public meetings and on his widely read blog.

Berlusconi's lawyers dismiss as "completely unrealistic" the possibility that, once his parliamentary immunity is lifted, he may face arrest over a series of other cases, including paying for sex with a minor.