By Gavin Jones
ROME (Reuters) - A populist movement led by a former comedian who calls Prime Minister Mario Monti "Rigor Montis" and dismisses mainstream parties as "political diarrhea" has rattled Italy's political establishment ahead of next month's local elections.
The 5-Star Movement, founded by 63-year-old Beppe Grillo in 2009, is successfully exploiting disaffection with politicians who are seen as corrupt and ineffectual, and polls suggest it could be a big winner at mayoral elections on May 6-7.
Increasingly popular at a time when support for mainstream parties is slipping, the movement - Grillo insists it is not a party - campaigns to clean up politics and business, promote clean energy and dismantle monopolies and privileges.
With his mane of white hair and beard, Grillo lays into left- and right-leaning parties alike, as well as the technocrat government of Mario Monti.
He made his name as a stand-up comedian in the northern city of Genoa, but Grillo is now better known for his vitriolic attacks against Italy's ruling classes, usually launched at packed rallies in the country's major cities.
Rudely nicknaming the understated Monti "Rigor Montis", Grillo says the current government represents the interests of banks rather than citizens.
At its first political test in 2010 the movement, which organizes itself through the Internet and social networks, won just 1.8 percent of the vote, rising to 3.4 percent at Milan's mayoral election the following year.
The travails of the Northern League, whose support is in free fall after leader Umberto Bossi resigned this month over a funding scandal, have served to increase Grillo's potential support however.
According to a poll published on Monday by the SWG agency, the 5-Star Movement, which has no party officials or structure - would now win 7 percent of the popular vote. That represents considerable support in a political landscape that is highly fragmented.
Grillo believes the momentum is unstoppable. At a rally in Genoa on Sunday he forecast its support would reach 20 percent and that it would bury rivals whom he said were "melting into each other like a sickening political diarrhea" due to their broad support for Monti.
It remains to be seen whether Grillo's current popularity will be reflected in success at the polls. But the other parties, clearly alarmed, are now attacking him rather than ignoring him as they have in the past.
On Sunday, Pier Luigi Bersani, leader of the centre-left Democratic party, warned that traditional parties risked being "swept away" by the rising appeal of populism. On the centre-right, former Foreign Minister Franco Frattini told Reuters that Grillo was "populist, extremist and very dangerous".
Nichi Vendola, leader of the Left and Freedom party, said Grillo's movement was "a mix of extreme right and extreme left policies which make it a disturbing phenomenon".
(Reporting By Gavin Jones; Editing by Andrew Osborn)