By Gavin Jones

ROME (Reuters) - A former leader of Matteo Renzi's ruling Democratic Party (PD) attacked the Italian prime minister on Monday and announced he would be voting against his plan to change the constitution in a crucial referendum next month.

The fierce criticism from Pier Luigi Bersani, who led the PD to a narrow victory at the last national election in 2013, widens divisions in the party and increases Renzi's difficulties ahead of the Dec. 4 referendum that polls suggest he may lose.

Bersani told reporters he felt "great bitterness" towards Renzi after his supporters had silenced internal critics with boos and chants of "get out, get out" at a party gathering in Florence on Sunday.

The chances of a major PD split appear to be growing whatever happens in the constitutional reform referendum, increasing uncertainty in the aftermath of the vote.

Bersani said that under Renzi the PD had become an "arrogant" party, where free debate had given way to a demand for blind obedience to the leader.

He said he would vote against Renzi's plan to reduce the role of the upper house Senate and cut the powers of regional governments, joining sides with other PD dissidents and all the opposition parties who are already lined up against the reform.

All but one of 37 opinion polls published over the last month have put the 'No' camp ahead.

Renzi loyalist Lorenzo Guerini, the PD's deputy leader, said Bersani's comments were "incomprehensible and disconcerting", adding that he had "often been disloyal".

DIVIDED PARTY

The PD's parliamentary party is divided between a majority of Renzi backers and a vocal minority of traditionalists like Bersani who say he has moved the party too far to the right.

One of Renzi's fiercest critics is another former PD leader, Massimo D'Alema, who served as prime minister from 1998 to 2000 and is vigorously campaigning against the constitutional reform.

With up to a quarter of voters still undecided, pollsters say the outcome of the referendum remains uncertain, but politicians are considering the scenarios if Renzi should lose.

Earlier this year the premier repeatedly said he would resign in the case of defeat. More recently he has declined to confirm that, saying discussion of his own future deflected attention from the merits of the reform.

The main opposition party, the anti-establishment 5-Star Movement, says that if he loses Renzi must "keep his promise" to quit and new elections should be held in early 2017, a year ahead of schedule.

Renzi himself appeared to countenance the possibility of defeat on Sunday when he said Italy did not need "a little technocratic government" to tide the country over until 2018.

That remark led many political observers to speculate that he still planned to step down if he loses on Dec. 4, but then to push for early elections in 2017.

(Editing by Tom Heneghan)