By Gavin Jones

ROME (Reuters) - Italians go to the polls on Sunday in local elections that will be the biggest test for Prime Minister Matteo Renzi since his center-left Democratic Party (PD) triumphed in a vote for the European parliament a year ago.

Some 22 million voters will elect new governments in seven of Italy's 20 regions and more than 1,000 municipalities following a campaign dominated by corruption allegations against one of Renzi's most prominent candidates.

The 40-year-old premier needs a convincing result to maintain momentum for labor and education reforms which have met fierce resistance from trade unions, the political opposition and the left wing of his own PD.

The center-left currently holds power in five of the seven regions. If Renzi maintains or improves on that tally against a divided opposition he will continue to look unassailable some 15 months after he come to office following an internal party coup.

The PD is unlikely to repeat its record performance at the European elections when it won 41 percent of the vote, but opinion polls, which have a poor track record in Italy, suggest it will remain easily the country's biggest party.

However, internal PD tensions erupted on Friday just two days before the election when parliament's anti-mafia committee released the names of 16 local election candidates suspected of corruption or organized crime links.

The so-called "impresentabili" or "unpresentable" candidates were from various parties but by far the most high-profile was Vincenzo De Luca, Renzi's candidate for president of the southern Campania region around the crime-ridden city of Naples.

Renzi accused the anti-mafia committee's chairwoman Rosy Bindi, who comes from the left-wing of his increasingly divided PD, of using the committee "to settle scores inside the party".

De Luca, a powerful local party baron, is accused of graft and also has a conviction for abuse of office which may prevent him from taking his seat even if elected. He denies wrongdoing and threatened to sue Bindi after he was included in the list.

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Renzi is hoping that signs of economic recovery after a three year recession will outweigh the furor over De Luca and other controversial candidates.

However, with the opposition aiming for a surprise result, Renzi said on Saturday the vote was pegged to local issues and would have "no consequences" for the stability of his government.

The anti-establishment 5-Star Movement, which polls say is the second largest party, believes it can capitalize on scandals surrounding mainstream parties and repeat the success of Spain's anti-austerity Podemos party in local elections last weekend.

The PD should win comfortably in its strongholds of Tuscany, Umbria and Marche in central Italy as well as in Puglia in the south-east, while the anti-immigrant Northern League is likely to keep power in the northeastern region of Veneto.

That means the main battle grounds are likely to be in north-western Liguria, where the PD vote may be weakened by a breakaway left-wing candidate, and in Campania, where De Luca is in a tight race with the center-right incumbent.

No exit polls will be released when polls close at 11 p.m. and results are likely to trickle in early on Monday.

(Editing by Crispian Balmer)