By Steve Scherer and Giuseppe Fonte

ROME (Reuters) - A referendum over Italian Prime Minister Matteo Renzi's flagship constitutional reform will be held on Dec. 4, the government said on Monday, with the fate of his administration likely to hinge on the outcome.

Renzi says the reform will bring much-needed political stability to Italy and has repeatedly pledged to resign if voters reject his proposals to reduce the role of the upper house, the Senate, and rein in the powers of regional governments.

But most recent opinion polls have put the "No" camp ahead and Renzi now refuses to be drawn on his future, saying he does not want the issue to dominate the referendum debate.

Renzi originally said he wanted to hold the ballot in early October, but he has pushed back the vote to one of the last practicable dates allowed to him by law to give the government more time to win over a skeptical electorate.

"We want a more stable and more simple country," he said in a statement after the date was announced. "Whoever desires change, lend us a hand."

The constitutional reform, which was approved by parliament in April after almost two years of fierce debate, effectively abolishes the Senate as an elected chamber and prevents it from bringing down a government via a vote of no confidence.

Under the current system, the upper and lower houses of parliament have equal powers and critics say this is one of the reasons why Italy has had 63 governments since World War Two, none of them strong enough to survive a full five-year term.

DECLINING FORTUNES

Renzi, who took office in February 2014, was highly confident he would win the vote six months ago when, encouraged by positive opinion polls, he staked his political career on the result. But the backdrop has since changed drastically.

The economy has unexpectedly slowed after growing just 0.7 percent last year, unemployment has remained stubbornly high and Britain voted in June to quit the European Union, sending a shockwave through the 28-nation bloc.

There has also been a souring of sentiment toward the 41-year-old Renzi. His swagger was welcomed by voters at first, bringing his Democratic Party more than 40 percent in the 2014 European Parliament election, but now many see him as arrogant and his approval rating has dropped to around 30 percent.

All opposition parties have lined up to denounce the constitutional reform, with some critics arguing that it strips Italy of vital democratic checks and balances put in place after World War Two to prevent the emergence of a new strongman.

"Italians will say 'No' and send him packing," said Renato Brunetta, the chief whip for Silvio Berlusconi's center-right Forza Italia party in the lower house.

The anti-establishment 5-Star Movement, the country's second-biggest party which won the mayorship of Rome in June, slammed Renzi for not consulting with the opposition on the referendum date.

Beppe Grillo, the comedian who founded 5-Star, announced at the weekend that he would return to running the group full-time after giving up management two years ago.

Opponents of the reform are also furious about the wording of the referendum question to be presented on the ballot sheet, which cites the more popular aspects of the reform but none of the less popular ones.

An opinion poll by Eumetra Monterosa published on Monday said 55 percent were set to vote "No" against 45 percent for "Yes". Another poll, by EMG Acqua, put "No" on 35.5 percent against 29.6 for "Yes", with 34.9 udecided.

By contrast, a survey by Ixe Institute also released on Monday gave "Yes" 38 percent and "No" 35, with 27 percent undecided.

Italy is not due to hold a national election until 2018, but one could be called sooner if Renzi's government were to fall and President Sergio Mattarella failed to find a stop-gap solution.

(Additional reporting by Gavin Jones; Editing by Crispian Balmer and Andrew Roche)