By Gavin Jones

ROME (Reuters) - Italian Prime Minister Matteo Renzi told thousands of party supporters gathered in central Rome on Saturday to step up their campaign in favor of a referendum on constitutional reform that could decide his political future.

With five weeks to go to the Dec. 4. referendum on his plan to reduce the role of the Senate and cut the powers of regional governments, Renzi is campaigning furiously to try to turn around opinion polls that suggest he may lose the vote.

"This is a choice between the future and the past," the 41-year-old premier told flag-waving members of his center-left Democratic Party (PD) at a rally in Piazza del Popolo.

"It is a chance for Italy to look at the future with a little bit more hope."

Renzi is appearing daily in television and radio interviews to try to rally support for the referendum, but with all the opposition parties, as well as a minority of his own PD lined up against the reform, he faces a tough task.

All but one of 26 opinion polls published this month has put the "no" camp ahead, with a lead ranging from one to nine percentage points.

Renzi asked his followers to set aside one evening a week to organize "a dinner, or a pizza, or a coffee" with wavering voters to win them over to the reform, warning that if it is rejected Italy will be "taken back a generation."

He said the PD would be stepping up its campaign in coming weeks, with more canvassing on the streets and telephone text messages to voters.

Renzi also appealed to PD traditionalists, including two of its former leaders, who have refused to back the reform and criticize him for moving the party to the right and for what they say is an authoritarian leadership style.

"It's time to end our internal divisions and quarrels," he told the crowd.

Earlier this year, Renzi repeatedly vowed to resign and quit politics if he lost the referendum, but over the last two months he has declined to confirm the pledge, saying debate over his own future deflected attention from the merits of the reform.

At the same time, he has sharply raised the pitch of his rhetoric in criticizing the European Union's fiscal rules which many Italians blame for their chronically stagnant economy.

He returned to the theme in Saturday's rally, saying he would continue to battle against "useless European rules."

He also ramped up the tone of a spat with Hungary's right-wing leader Viktor Orban, saying Orban should "wash his mouth out before speaking about Italy."

This week Renzi said Hungary should have its EU funding cut if it refuses to take in a fair quota of migrants, to which Orban replied that Renzi was "nervous" over Italy's public finance problems and the migrants arriving on its shores.

(Reporting By Gavin Jones; Editing by Richard Balmforth)