Ex-premier Enrico Letta announced he was running to be the new leader of the centre-left Democratic Party (PD) in a video posted on Twitter on Friday.
Lazio Governor Nicola Zingaretti recently quit as leader, complaining of an "unending stream" of criticism from within the party.
The party has fallen sharply in the opinion polls since he resigned and several PD bigwigs asked Letta to take over.
"Monday I would never have imagined standing to lead the party that I helped found and which today is in a deep crisis," said the 54-year-old, who had taken a few days to consider whether he wanted to run.
"I am doing so for the love of politics and passion for democratic values.
"I don't seek unanimity. I seek truth in relations among ourselves in order to emerge from the crisis and look long term.
"I will open a debate in all the PD sections".
Letta had stepped back from frontline politics after then-PD-leader Matteo Renzi brought down his broad coalition government in 2014 in order to take over the helm of government himself.
Renzi has since split from the PD to form his own centrist party, Italia Viva (IV).
Letta thanked his predecessor Zingaretti, saying he remained bonded to him by "deep friendship and great esteem".
Zingaretti said he thought Letta was "the right person" to take over the PD.
"I'll do my best as governor to show the good government of a new centre left," Zingaretti continued.
"Ill make my contribution via ideas.
"If I quit, it was to help, out of love for the party and for Italy.
"I hope that the season of rows is over and the season of the PD being a lead player has begun".
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