By Gavin Jones
ROME (Reuters) - Italians went to the polls on Sunday in local elections that are 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 around 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, education and constitutional 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 improves on that tally against a divided opposition he will continue to look unassailable some 15 months after he come to office.
Opinion polls, which have a poor track record in Italy, suggest the PD will remain easily the biggest party. However, internal party tensions erupted on Friday when parliament's anti-mafia committee released the names of 16 local election candidates suspected of corruption or organized crime links.
By far the most high-profile of the "impresentabili" or "unpresentable" candidates was Vincenzo De Luca, Renzi's candidate for president of Campania, a crime-ridden southern region around the city of Naples.
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 the anti-mafia committee's chairwoman, Rosy Bindi, for putting him on the list.
Renzi accused Bindi, who comes from the left wing of the PD, of using the committee "to settle scores inside the party".
Opposition parties are hoping to capitalize on the furor.
The anti-establishment 5-Star Movement, which polls say is the second largest party, hopes it can emulate the success of Spain's anti-austerity Podemos party in local elections last weekend.
The anti-immigrant, anti-euro Northern League expects to gain from the unprecedented numbers of African and Middle-Eastern migrants that have arrived on Italy's shores this year. More than 5,000 have been rescued in the Mediterranean this weekend alone and are being brought to southern Italian ports.
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 Northern League is likely to keep power in the north-eastern 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.
By 7 p.m. (1700 GMT) around 39 percent of the electorate had voted in the elections for the regional governments and 49 percent in mayoral contests, according to partial interior ministry data. When polls close at 11 p.m. exit poll results will be issued for the regional votes in Liguria and Campania.
Projections based on the actual vote count will be released at regular intervals during the night.
(Editing by Crispian Balmer)