Yet, mutual anymostity still runs deep among the parties at the local level, and some of 5-Star's most prominent players have rebelled against the PD coalition, fomenting internal party tensions.
Members of the anti-establishment 5-Star Movement backed Friday a local alliance with the center-left Democratic Party (PD) in an online vote that could strengthen the new government and change the shape of Italian politics.
Following the recommendation of party chief Luigi Di Maio, 61 percent of 5-Star members voted in favor of fielding a joint candidate with the PD in an election next month to pick a new governor of the central region of Umbria.
The vote marked a reversal of 5-Star's traditional rejection of election pacts and the repercussions will extend far beyond Umbria, a land-locked region of 900,000 inhabitants in central Italy.
By joining forces in Umbria, traditionally a PD stronghold, 5-Star and the PD hope to halt a long streak of victories by the center-right, which has triumphed in seven regional ballots since the national election in March 2018.
Success at the Oct. 27 Umbria vote would probably pave the way for similar alliances in the next few months in the regions of Emilia-Romagna in the industrial north, and Calabria in the poorer south.
In the medium term, analysts say this would create the conditions for 5-Star and the PD to become a stable alliance that would contest future local and national elections against a conservative bloc led by Matteo Salvini's hard-right League.
5-Star governed Italy with the League until last month, when Salvini pulled the plug on the coalition in a vain attempt to trigger elections he expected to win. Instead, 5-Star formed a new government with the PD, pushing Salvini into opposition.
However, the path to stable 5-Star/PD cooperation may not be anything but easy.
Driven by a common desire to block Salvini, the traditional adversaries overcame their differences to form a national government, but fielding joint candidates in regional and mayoral elections requires a deeper level of cooperation.
On Thursday Alessandro Di Battista, considered a potential future 5-Star leader, warned his colleagues not to trust the PD, calling it Italy's most pro-establishment and "dangerous" party and "the most hypocritical party in Europe."
Even after Friday's vote in favor of the Umbria pact with the PD, the two parties still need to find a mutually acceptable candidate for governor. Preliminary talks have made little progress, running up against vetoes and local rivalries.
5-Star's pre-condition for any local deals is that candidates must come from so-called "civil society", meaning they cannot be career politicians or members of either party.