By Isla Binnie

ROME (Reuters) - Widening divisions in Italy's ruling Democratic Party (PD) may reduce the chance of the early elections its leader Matteo Renzi has been calling for since he was toppled as prime minister last month.

Rivals in the PD threaten to break away and two opinion polls this week suggest they would take lots of votes with them, sharply reducing Renzi's chances of returning to power.

Renzi, who stepped down as premier when Italians threw out his proposals for constitution reform in a referendum, has been pushing for a vote by June, around a year ahead of schedule. Italy's main opposition parties also want a snap vote.

But a sizeable minority of Renzi's critics in the PD, who say he has moved the party too far to the right, want to challenge his leadership before any return to the polls.

Massimo D'Alema, a former prime minister and one of Renzi's fiercest critics, said if early elections were called he and his followers would "consider ourselves free" and split off.

A poll published by Tecne on Monday said the PD would win 29 percent of the vote. But it would slump to 20 percent if D'Alema and fellow traditionalist Pier Luigi Bersani formed an alternative left-wing movement, which would take 14 percent.

The poll said the anti-system 5-Star Movement is Italy's most popular party, on 30.5 percent.

5-Star's appeal to Italians tired of corruption and economic stagnation shows no sign of diminishing despite the difficulties plaguing Rome's Mayor Virginia Raggi, who holds one of the party's most high-profile public roles.

Another poll, by IPR, put the PD and 5-Star neck-and-neck on 30 percent and said the PD would fall to 22 percent if D'Alema and Bersani were to go it alone.

However, both polls said no party would win a parliamentary majority under the current proportional electoral system, probably dooming Italy to the sort of unwieldy coalition long blamed for political instability.

D'Alema and his allies say it would be irresponsible to topple current Prime Minister Paolo Gentiloni, a PD member who is widely seen as Renzi's proxy, without first changing the electoral law.

Having gone to ground after the referendum defeat, the fast-talking Renzi reignited a public relations campaign last week, and on Monday raised the prospect of a return to government, pledging to fix notoriously high taxes if elected.

(Additional reporting by Gavin Jones; Editing by Tom Heneghan)