More than 1,300 municipal councils will be elected in a two-round ballot to be completed on June 19, with the primary focus on the major cities of Bologna Milan, Naples, Turin, and especially the capital, where the populist,
anti-establishment Five Star is heading the race for the mayor's seat.
Rome has been without an elected leader since last October, when Ignazio Marino, a member of Renzi's centre-left Democratic Party, was forced to quit over an expenses scandal.
That episode and a much bigger, unrelated scandal over organised crime's infiltration of City Hall have bolstered Five Star's Virginia Raggi.
She goes into Sunday's vote with polls indicating she could secure around 30 percent of first round votes. The PD's Roberto Giachetti was trailing on around 24 percent.
Losing Rome would not augur well for Renzi four months before he puts his position on the line in a referendum on constitutional reforms designed to end decades of gridlock in parliament.
And the setback would be even greater if, as looks possible, the PD candidate in Milan is also defeated.
Five Star meanwhile are hoping that success in Rome will give them the platform they need to transform themselves into Italy's principal opposition in the run-up to national elections due by June 2018 at the latest.
"I think for Five Star the best outcome in Rome would be to lose narrowly in the run-off, they could talk that up as a glorious defeat and continue to be in opposition," said Piergiorgio Corbetta, an academic specialist on the
"If they win they have to make the move from being a protest party to proposing things and it will only take six months for them to show they are genetically no different from the other parties they accuse of corruption and
dishonesty," he said.
With former premier Silvio Berlusconi now a fading figure on the national stage, Italy's right is being reshaped and the battle for leadership is being played out in Rome.
Giorgia Meloni, a candidate put up by one of several grouplets that emerged from Italy's neo-fascist movement, is being backed by the anti-immigrant Northern League, whose leader Matteo Salvini wants to united all the right
But Berlusconi has backed another candidate, Alfio Marchini, having told the pregnant Meloni that the role of mayor was not compatible with motherhood.
PD candidates are comfortably ahead in the polls in Turin and leftist bastion Bologna while the outgoing mayor of Naples, an independent who has been a vocal critic of the government, is also expected to be returned to
Perhaps concerned about the impact of losing both Rome and Milan for the momentum of his reform programme, Renzi has played down the significance of the local elections.
"The municipals are about mayors, the people whose job it is to repair the streets, not the government of the country," he said recently. Some 16,000 polling stations open at 7.00 am (0500 GMT) and close at 11.00
There are concerns the turnout could be low with millions of Italians enjoying a long holiday weekend as a result of Republic Day falling on Thursday.