Premier Paolo Gentiloni's PD also held a directorate meeting on Friday as part of the process of setting the rules for the congress.
Ex-premier Matteo Renzi is being challenged for the party's leadership by Justice Minister Andrea Orlando, Puglia Governor Michele Emiliano and Carlotta Salerno, a Turin party chief.
Renzi supporters were said to be pushing for leadership primaries to be held on April 9 but the other candidates wanted more time.
A group of left-wingers headed by former PD chief Pier Luigi Bersani broke away from the PD this week due to differences with Mr Renzi.
The former PD leader, who is on a fact-finding trip to the United States about growth and job creation, resigned as premier in December after almost three years in office following the rejection of his flagship constitutional reform in a referendum.
Mr Renzi, who has been accused of arrogance, trying to make the PD a personalised party and moving it too far to the centre, was replaced as premier by former foreign minister Paolo Gentiloni.
Polls released Friday by the Ixè Institute for State broadcaster RAI said that the PD had lost 2.3 percentage points of support because of the split, taking it down to 28.1%.
The results mean its lead over the anti-establishment 5-Star Movement (M5S) is now extremely narrow, with M5S gaining 0.8 of a point to reach 27.8%.
With ANSA
