* Disappointing Monti fades ahead of Sunday vote
* Experts say crucial Senate race too close to predict
* Unstable result, weak government could destabilise euro zone (Adds Grillo rally)
By Steve Scherer and Barry Moody
ROME, Feb 22 (Reuters) - Italy's election campaign drew to a close on Friday with the weak performance of outgoing premier Mario Monti key to a deeply uncertain and potentially unstable result.
Political leaders were holding their final rallies before a campaigning ban ahead of two days of voting on Sunday and Monday, and analysts said the result was too close to call.
Grabbing headlines and tapping into a national mood of disillusion with politicians and economic austerity, comedian turned wildcard campaigner Beppe Grillo whipped up a crowd of half a million in Rome, telling established parties: "Get out!"
"This vote is not at all certain," said one pollster who asked not to be named because of a ban on the publication of opinion surveys in the fortnight before voting. "One percentage point either way could lead to chaos or a clear winner."
Analysts are divided over whether centre-left leader Pier Luigi Bersani, who was five points ahead two weeks ago, will be able to form a stable majority capable of pursuing the economic reforms that an uncompetitive Italy needs to exit recession.
Bersani is thought now to be just a few points ahead of centre-right leader Silvio Berlusconi but is still seen with a good chance of the winner's bonus of parliamentary seats that will give him comfortable control of the lower house.
However, the election will revolve around the much more complex Senate race, where winners' bonus seats are awarded on a region-by-region basis.
The centre-left and centre-right are close to a draw in several battleground regions, including industrial powerhouse Lombardy, which returns the most senators. Berlusconi formed an alliance with the federalist Northern League because of the importance of populous northern regions in the Senate race.
Monti, a technocrat former European Union commissioner and economics professor who has led a lacklustre centrist campaign, is believed to be fading but analysts disagree about the implications.
Electoral expert Roberto D'Alimonte expressed concern that Monti may fall below the 8-percent threshold to win Senate seats in some regions and said this could be disastrous for Italy.
"What is really important is that Monti does not fall under eight percent in several regions," he told Reuters. "The future of Italy and the euro zone depend on this variable."
The most likely - many say the most stable and pro-reform result from the election - appears to be a governing alliance between Bersani and Monti, which would require the outgoing premier to win enough senators to boost the centre-left.
Bersani has suggested he wants to form an alliance with Monti to give his government credit with the financial markets and a strong law-making mandate, even if the centre-left wins a majority on its own.
However, some other analysts believe that Monti is fading particularly in the "red centre" of Italy where the votes he loses will go to Bersani's Democratic Party (PD), increasing the chances of a strong centre-left government which will be able to drive a hard bargain with a weakened Monti over policy.
One of the unstable scenarios after the vote would be a government ranging from leftists to a strong, centrist Monti; that could become entangled in constant policy disputes.
"If Monti doesn't do well in the Senate, it will be to the benefit of the Democratic Party," Maurizio Pessato, vice president of SWG pollsters, told Reuters.
"The key with this electoral system is not Monti, it's the PD winning Lombardy ... If the centre-left wins the Senate outright, it will make an alliance with Monti from a position of strength. There will be a government and stability."
Pollster Nicola Piepoli told Reuters: "Bersani will gain. He is the principal beneficiary of Monti's crisis.
"This makes it more likely that he can win in the Senate."
Whatever government emerges from the vote, it will need to address Italy's deeply uncompetitive economy, in its longest recession for 20 years and effectively stagnant for two decades.
The biggest danger for Italy and the euro zone is that the election produces a weak government incapable of decisive action, spooking investors and igniting a new debt crisis.
The disappointing performance by Monti, darling of the markets and of other European governments, has been a feature of a bitter campaign that has also seen a stunning resurgence by Berlusconi and major success for Genoese comic Grillo, now a potential third force in parliament, who has exploited huge public anger over the economy and a rash of corruption scandals.
The latter two factors have whittled away the centre-left's commanding 10-point lead since December and made the election result increasingly uncertain.
On Friday evening, some 500,000 or more attended the final rally on Grillo's "Tsunami Tour" of the country in Rome's San Giovanni square. The charismatic comedian whipped the crowds into a frenzy as he railed against the establishment, as well as at foreign leaders including German Chancellor Angela Merkel.
Berlusconi cancelled a planned appearance at a rally in Naples, citing an eye problem. But in an interview on his Canale 5 television channel he made a last-minute promise to raise the lowest retirement pensions to help vulnerable old people.
Monti replaced Berlusconi in November 2011 after the media tycoon had taken Italy towards a perilous debt crisis and became embroiled in a series of scandals. He shored up Italy's credit abroad and sharply reduced the country's borrowing costs.
But he has made a poor candidate in his first election. Experts say he was badly advised, including by U.S. consultants who have worked for President Barack Obama, and clumsily attempted to join in with political mud slinging instead of retaining the aloof, professorial demeanour that has marked his time as prime minister.
"If he was seen by Italians as a robot, then he should have been a robot in the election campaign," said Antonio Noto, head of the IPR polling company. "If you change behaviour then you are not seen as genuine."
Monti was also an easy target for both Berlusconi, whose campaign centred on attacking the prime minister's hated new housing tax and Grillo who nicknamed him "Rigor Montis". (Additional reporting by Paolo Biondi, Catherine Hornby and Naomi O'Leary; Writing by Barry Moody; Editing by Anna Willard and Alastair Macdonald)