The proposed voting system is backed by three of the country's four largest parties, with the center-left government looking to rush it onto the statute books ahead of elections, which are due by May 2018.
Five-Star supporters protested in front of parliament as the Chamber of Deputies approved two confidence motions by a wide margin. A third such vote is scheduled for Thursday ahead of a final ballot in the lower house on the disputed bill.
Unlike the current rules, the new system would allow the formation of multi-party coalitions before the ballot, a factor likely to hurt 5-Star, which is topping most opinion polls and refuses to join alliances.
'They want to take away our right to choose,' said Nicola Zuppa, 45, who said he had paid 175 euros ($200) to travel from Padua in northern Italy to take part in the protest, which drew up to 2,000 people in the heart of Rome.
5-Star Movement supporters protest in front of Montecitorio government palace in Rome, Italy, Oct. 11, 2017.
The use of multiple confidence motions allowed the ruling coalition to truncate discussion on the bill and sidestep dozens of planned secret votes on various amendments. The reform still needs the approval of the upper house Senate.
'If you allow the electoral rules to be changed again so that the scum of the country rises to the top yet again, it will be your children who pay the price,' 5-Star's founder Beppe Grillo wrote on his blog on Wednesday.
President Sergio Mattarella, the only figure with the power to dissolve parliament, has called for new voting rules because the current system is very different for the upper and lower houses, meaning it could throw up conflicting majorities.
All previous attempts to harmonize the rules have failed, most recently in June when dissident deputies used a secret vote to upend part of the proposed legislation.
Former Prime Minister Matteo Renzi's Democratic Party (PD) drafted the latest version, which is supported by right-wing parliamentary rivals Forza Italia (Go Italy!) and the Northern League. Five small parties are also backing the proposed law.
Five-Star estimates that the new rules could cost it up to 50 seats in the Chamber of Deputies and potentially scupper its chances of being the largest group in parliament after the vote.
Mattarella is expected to give the formation that gets the most seats the first crack at forming a government. The PD has denied trying to stymie the 5-Star's chances.
'No one is preventing (5-Star) from making alliances if they want to,' said Ettore Rosato, the parliamentary party leader of the PD who has put his name to the reform. 'If they don't want to do them, they can continue to be an isolated party.'
Analysts say the new electoral system looks unlikely to throw up a clear parliamentary majority, with opinion polls showing the center-left, center-right and 5-Star splitting the vote three ways.
Such a result could lead to the creation of a grand coalition that would need to span the political divide.
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