By Gavin Jones
ROME (Reuters) - Voters in Italy's largest cities go to the polls on Sunday in local elections that could weaken Prime Minister Matteo Renzi who is struggling to spur economic growth and create jobs.
Renzi's Democratic Party remains Italy's main political force, polls show, but its support has been ebbing for a year and the PD's candidate is a clear favorite in only one of the four main cities up for grabs - Rome, Milan, Naples and Turin.
If, as seems likely, no candidate wins more than 50 percent of the votes on Sunday, run-offs between those who come first and second will be held on June 19.
Some 13 million Italians, or a quarter of the adult population, are eligible to vote for mayors of more than 1,300 towns and cities, though media attention has focused on the major centers.
Renzi has devoted little time to the elections and only this week has he begun campaigning on behalf of his candidates.
"He knows there is a significant risk that things won't go well for him so he doesn't want to be closely associated with what could be a negative outcome," said Roberto D'Alimonte, a political science professor at Rome's LUISS University.
Defeat in three or more of the big cities would weaken Renzi and galvanize his opponents, but would "certainly not trigger the fall of the government," said D'Alimonte.
Renzi, who took office in 2014, is criticized by leftists in the PD who say he has moved the party too far to the right.
The Renzi-backed candidate is expected to win in Turin and is slight favorite in Milan. Victory in the capital Rome or Naples would be positive surprises.
Rome offers the chance of a huge breakthrough for the anti-establishment 5-Star Movement, which hopes to capitalize on anger toward the main parties on the left and right which have been embroiled in a spate of corruption scandals in the city.
Five-Star's candidate Virginia Raggi, a photogenic 37-year-old lawyer, hopes to become the city's first woman mayor and was ahead before the publication of opinion polls was banned 15 days before the vote, as dictated by Italian law.
Milan is probably the key contest for Renzi. His chosen candidate Giuseppe Sala, the former head of the Expo world fair held in the city last year, was expected to win comfortably but now looks seriously threatened by a moderate center-right rival.
Defeat for the PD would be little short of a disaster in Turin, where incumbent mayor Piero Fassino, one of the party's best-known national figures, was well ahead in the polls.
However, if Fassino fails to clinch a first-round victory he may face a close run-off if voters of the other main parties gather behind the 5-Star candidate, seen as his main rival.
The southern port city of Naples, on the other hand, is considered a lost cause for the PD and is widely expected to be held by the leftist incumbent Luigi de Magistris, an independent former prosecutor who has declared Naples a "Renzi-free zone."
(Editing by Robin Pomeroy)