By Barry Moody

ROME (Reuters) - Silvio Berlusconi's resurgence and the rise of a foul-mouthed populist comedian have thrown Italy's weekend election wide open, with deep uncertainty over whether the poll can produce the strong government the country needs.

Italy, the euro zone's third biggest economy, is deep in its longest recession for 20 years. Successive governments have failed to revive an economy stagnant for two decades.

Public fury over record unemployment - especially among the young - tax hikes and economic pain, combined with a recent rash of high-level corruption cases has fanned support for comic Beppe Grillo's anti-establishment 5-Star Movement.

A skilful Internet user, Grillo has been the most active on the hustings, touring Italy in a camper van on a "tsunami tour", shouting himself hoarse with obscenity-laced insults at a discredited political class, winning roars of approval from large crowds.

Some analysts say his could be the third biggest single party in the February 23-24 vote with around 20 percent support, ahead of Berlusconi's People of Freedom (PDL).

Berlusconi, 76, spent most of 2012 in the shadows - undermined by a lurid sex scandal - after he was ignominiously bundled out of power and replaced by technocrat Mario Monti in November 2011 as Italy faced a grave debt crisis.

Monti imposed austerity policies and brought borrowing back under control, winning plaudits from European partners.

But the billionaire media magnate burst back in December in an extraordinary blitz of television appearances, belying his age, that halved centre-left Pier Luigi Bersani's 10-point poll lead over the former premier's centre-right coalition.

Berlusconi, a born showman and master communicator, backed by a television empire, has won back voters by attacking a hated housing tax imposed by Monti and offering to pay it back, something his opponents say is an impossible vote-buying trick.

Bersani, a worthy but dull former minister, and his Democratic Party have seemed stuck in their tracks, complacent about their lead and unable to respond dynamically to the threat from Berlusconi and Grillo.

Monti has disappointed European partners and investors who would like to see him return to office to continue his reform agenda. Analysts said on Thursday his centrist support was draining and he may fall below 12 percent.

CENTRE-LEFT EXPECTED TO WIN

Pollsters still believe the most likely outcome is a centre-left government headed by Bersani and backed by Monti, but the rise of Grillo and Berlusconi, and the outgoing premier's weak performance, are causing jitters about whether this will produce a strong government committed to reform.

Latest published polls before a legal blackout on February 9 showed the centre-left about 5 points ahead and analysts believe this still holds, with perhaps a slight decline in support.

Italy may lose momentum on reforms vital to revive growth if no clear winner emerges from the election, credit ratings agency Standard & Poor's said on Wednesday.

The spread between the yield on safe-haven 10-year German bonds and their Italian equivalent widened by 14 basis points on Thursday in a sign that hitherto relaxed investors were slowly waking up to the risk of a weak government.

Bersani's opponents also say he would be unable to agree on reforms with Monti and would be hobbled by trade unions and leftists, something party officials deny.

Grillo's owes much of his success to rage at the failure of lawmakers to keep their promises to transform the economy, change a badly flawed electoral law, cut the privileges of a pampered political class or combat rampant corruption.

Critics say his policies are vague and impractical and hardly anything is known about the 5-Star candidates, whom Grillo keeps out of the limelight and under iron control.

A bloc of more than 100 5-Star members of parliament, or a sixth of the lower chamber, could be deeply disruptive and further delay reforms.

"The 5-Star Movement is providing an outlet for rage and frustration. The traditional parties are incapable of indicating any other course," said author Beppe Severgnini in a front page editorial in the Corriere della Sera daily on Thursday.

Massimo Franco, one of Italy's top political analysts, told Reuters: "Grillo is the most visible sign of a crisis in the political system, the system that people are fed up with...this is the end of a system not of a government."

Ironically, however, a late surge by Grillo may hurt Berlusconi more than the left and could increase the chances of a more stable Bersani-Monti government.

Analysts say that, helped by Grillo's rise at Berlusconi's expense, Bersani may even be able to win undisputed control of the Senate or upper house, something which has been in doubt.

Under Italy's electoral law, Bersani will win comfortable control of the lower house even if he is only just in front of Berlusconi because of a hefty winner's bonus.

In the Senate the bonus is allocated by region, hence the centre-right target is to stymie a centre-left government in the upper house, which has equal law-making power to the Chamber of Deputies.

Bersani has said he would like to ally with Monti even if he wins alone, to give his government stronger cover in the markets and underline his declared commitment to pro-European reforms. But he refuses to abandon his leftist ally Nichi Vendola as the price of a post-election pact.

Bersani's economy spokesman, Stefano Fassina, told Reuters this week that the centre-left was committed to urgent economic reforms, headed by streamlining a bloated public sector and making radical changes in a complex judicial system that is a disincentive to investment.

Monti would not be able to refuse to enter a governing alliance that guaranteed stability, reform and a bulwark against dangerous populism, he said.

(Writing by Barry Moody; Editing by Paul Taylor)