Like many Sicilians, Agatino Zappala, who runs a delicatessen in the city of Catania, voted for the 5-Star Movement at Italy's last national election in 2018 but will switch his allegiance to the right at this month's vote.

The trend, reflected in polls, could give the conservative bloc a landslide victory on the island at the Sept. 25 ballot and help pave its way to power nationwide.

Sicily has often been a bellwether for Italy's national electoral shifts.

"I supported 5-Star but it was a wasted vote," says Zappala as he served customers near Catania's colourful open-air market in the shadow of the towering Etna volcano.

The 5-Star Movement has haemorrhaged most of its support since it took 32% of the national vote in 2018 on an anti-establishment ticket that defied left-right labels and promised to shake up Italian politics, something its former voters say it has failed to do.

Zappala, who has not yet decided which of the right-wing parties he will vote for, blames 5-Star above all for its decision to form a government in 2019 with its old foes the centre-left Democratic Party (PD), a group he deeply dislikes.

"To me, the PD is the worst possible ally," he said.

RIGHTIST CLEAN-SWEEP?

In 2018, 5-Star won all the 28 first-past-the-post seats up for grabs in Sicily, where an unemployment rate around twice the national average and poor public services fuelled grassroots support for what was widely seen as a protest movement.

This time a conservative bloc led by Giorgia Meloni's Brothers of Italy is on track to take all the first-past-the-post constituencies won by 5-Star four years ago, a study by polling firm YouTrend shows.

Italy's election system allocates two thirds of seats by proportional representation, and the rest by British-style first-past-the-post.

"The south, and Sicily as part of the south, can make a difference ... and confirm the estimates of a clear centre-right majority in the next parliament," YouTrend's head Lorenzo Pregliasco told Reuters.

YouTrend's study on Sicily, seen by Reuters, shows 5-Star is polling at between 11% and 21%, steeply down from the nearly 49% it got in 2018.

That could still make it the largest single party on the island, but it is running alone, making it hard to compete in the first-past-the-post seats where its rivals have joined forces in alliances.

The right-wing bloc, comprised of the League and Forza Italia as well as Brothers of Italy, is seen scoring between 42% and 52%, after getting around 32% four years ago. The PD-led centre-left is estimated at 20-30%.

Many southern first-past-the-post seats would be closely contested, Pregliasco said, if the PD and 5-Star had not broken off their previous alliance in July, over 5-Star's decision to withdraw support for Mario Draghi's government.

PROTEST AND DISILLUSION

Sara Gentile, politics professor at Catania university, said 5-Star has lost appeal for Sicilians because it has become increasingly institutional, while the island's voters tend to pick those who claim to be new and promise to redress injustice.

"The 5-Star can no longer provide guarantees to those who cast a protest vote ... while Meloni is trying to represent the people by using far-right language," she said. "There is a powerful right-wing appetite here."

Some southern politicians have also switched to the right, such as Giovanni Grasso, who was 5-Star's candidate for mayor of Catania in 2018 and joined Brothers of Italy last year.

"I have always been right-wing ... it was a return to my origins," he said, citing a lack of organisation and of political leadership among reasons he left 5-Star.

Grasso hopes the right wins big at the election but believes 5-Star's flagship "citizens' income" poverty relief scheme will help his former party to stay afloat in Sicily, where many are on the breadline.

Around 500,000 of Sicily's 4.8 million residents receive the citizens' income, making it the second largest beneficiary behind the Campania region around Naples. Some 120,000 of them live in Catania and around, official data show.

While angry Sicilians often cast a protest vote many others are too resigned and disillusioned to vote at all, leading to traditionally high abstention rates.

Giuseppe Costanzo, who works at the fish market in the centre of Catania, plans to stay at home on Sept. 25 just as he did in 2018.

"The left and the right do not exist anymore, it is just a game for the politicians," he said.

(Editing by Gavin Jones and Frank Jack Daniel)