Renzi, who stepped down in January after losing a key referendum, said he would run to win back his post as party secretary, which would put him in pole position to become prime minister once more should the PD win approaching national elections.
The showdown with his many enemies looked set to be fierce; rebels on the far-left have been threatening to split off, with a potential fallout for the government and PD Prime Minister Paolo Gentiloni.
"Schism is one of the worst words, only one is worse and that's the word blackmail. It is unacceptable for a party to be blocked by the diktat of the minority wing," he said.
"You have the right to defeat us, not eliminate us. That's the heart of a democracy," he added.
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The congress to elect the new PD party secretary will be held in June.
Gentiloni's government hopes to carry on until the natural end of the legislature early next year.
The 42-year old called for the party to "move forwards united," saying he and the others in the majority asked "with our hearts in our hands" for the rebels to stay.
But former PD leader Pier Luigi Bersani, who still has core support among traditional PD party voters, dismissed his appeal.
"We are at very delicate point. There are those who think we are heading for a crash which will hit not just the PD but Italy," he said.
Bersani blamed Renzi for "putting up a wall" and feared the run-up to the congress the minority's concerns would "not be open to discussion".
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