Luigi Di Maio said Tuesday that in "the next hours" the nation will learn the lineup of the coalition Cabinet ministers in what would be Premier-designate Giuseppe Conte's second government. Together, the two parties would command a slim majority in Parliament, where Conte must win confidence votes in order to govern.
With infighting plaguing both the 5-Stars and the Democrats, the vote in Parliament could be very close.
Supporters of Italy's anti-establishment 5-Star Movement are voting in an online ballot that will determine whether the party forms a new government with the center-left opposition Democrats.
Party activists are voting on whether to go ahead with a coalition with the Democrats, who are traditional foes, in a ballot that ends at 6 p.m. (1600 GMT) Tuesday. Results are expected shortly afterward.
A "yes" vote would pave the way for premier-designate Giuseppe Conte to form a new cabinet after the 5-Stars' previous coalition with the anti-migrant League collapsed. A vote against the new coalition would risk taking Italy into a new election that could strengthen the League.
The 5-Stars are deeply divided over a tie-up with the Democrats, as the two parties remain far apart on key policies.