"Our program points are clear, if they do not fit into the government program, we will not be able to start, and then it will be better to return to the polls as soon as possible," Di Maio said Friday.
"Either we agree to carry it out, or we can not go further," he still threatened, after meeting with the head of the government appointed Giuseppe Conte.
Di Maio notably rejected an amendment to the "security decrees" promulgated by the outgoing Minister of the Interior and his former ally the leader of the sovereignists Matteo Salvini.
"The problem of immigration is serious, concrete and it must be confronted competently in the respect of sensitivities expressed by the public opinion," he said, aware that much of the M5S approves the security hardening promoted by Mr. Salvini, during the 14 months of alliance M5S-League.
A senior PD leader, Graziano Delrio, described Di Maio's ultimatums in Conte as "unacceptable", recalling that the Social Democrats "pledged to loyally support Mr. Conte's efforts" to form a government.
Previously, the boss of the DP, Nicola Zingaretti, had listed, to the address of Mr. Conte, the "main novelties" of a common program M5S-PD: a reduction of the taxes for the means and low wages "in order to stimulate consumption ", a plan for employment, promoting the digital economy and the possibility of" making free the training of children from modest families from nursery to university ".
Conte, appointed Thursday by President Sergio Mattarella to try to form an M5S-PD government, consulted for two days the different political forces ending Friday with the PD and the M5S with whom he must build a new government team.
He is expected to present the government's composition to President Sergio Mattarella by next Tuesday or Wednesday before taking an oath.
In addition to the distribution of ministerial portfolios, several unknowns remain, including the assignment of the post of Deputy Prime Minister, which Di Maio would like to keep. According to the political scientists, very weakened, "he fights for his survival".
The strategic positions of the Economy and the Interior are also at the heart of the discussions.
The Five Stars will also submit the coalition agreement to the vote of their activists on their participatory platform "Rousseau". This online consultation of the 100,000 registered will take place in the coming days, according to the media.
Matteo Salvini had declined the invitation of Mr. Conte and sent simple representatives.
On 8 August, the right-wing leader broke the coalition formed by the League he leads and the Five-Star movement, as polls predicted that the League, Forza Italia – the center-right party Former Prime Minister Silvio Berlusconi – and the far-right party Brothers of Italy together could get enough votes in early elections to form a government majority.
After this failed gamble, for the next few months, Salvini's battle plan is clear: to go on campaign to bring down the government and get elections. "In a few months, a year or two of opposition, we will come back," he promised, announcing all-out mobilizations including a "day of Italian pride", on October 19 in Rome.
He has already ruled out that the League forms a future coalition with Forza Italia. "We do not need anything or anyone," he said during a rally Friday at Conselve, near Padua, in his stronghold in northern Italy.