ROME (Reuters) - Italy’s traditional political parties have pledged to back a new electoral law this week that is seen penalizing the anti-establishment 5-Star Movement ahead of next year’s national election.

The lower house of parliament is due to start voting on Tuesday on an electoral law supported by the country’s main right- and left-wing parties. Unlike the current system, it would allow the formation of broad coalitions before a vote, a factor likely to hurt 5-Star, which refuses to join alliances.

“The only aim (of the law) is to allow others to gang up on us,” 5-Star lawmaker Danilo Toninelli said on Sunday, a day after a parliamentary committee signed off on the new proposal.

For the traditional parties, “the important thing is to stop the 5-Star,” he said.

A national vote is due to be held during the first half of 2018, between March and May.

Polls show Matteo Renzi’s ruling center-left Democratic Party (PD), which proposed the new law, is neck-in-neck with the 5-Star. But if the PD runs in alliance with smaller centrist and left-wing parties, together they would likely surpass the 5-Star, now led by the 31-year-old Luigi Di Maio.

The new law may be even more beneficial to Silvio Berlusconi’s center-right Forza Italia (Go Italy!) party and the far-right Northern League, which also back the bill. In a coalition, polls show they could win the most votes.

“The wind is blowing in the sails of the center-right and of Forza Italia,” the lower-house speaker of Berlusconi’s party, Renato Brunetta, said.

But polls give neither Renzi’s center-left or Berlusconi’s center-right enough votes to govern alone, suggesting a right-left alliance may be the only way to form a government after the election.

The bill could pass as early as this week in the Chamber, after which it would go to the Senate for approval. The risk is that it could be sunk by dozens of secret votes in which parliamentarians can break with the official party line.

Secret voting is what killed a previous election law proposal in June that initially had been supported by the 5-Star Movement.

The 5-Star catapulted into parliament in 2013 with nearly a quarter of the national vote, but it has struggled to further expand its support. It has vowed to rescue Italy from the clutches of what it says is a corrupt political system.

Five-Star’s policies include the introduction of universal income support for the poor, rewriting European Union public spending limits, and phasing out fossil fuels. It now says holding a referendum whether to dump the euro, which had been a pillar of its platform, is not a priority.

The proposed election law would distribute almost two-thirds of the seats in parliament on a proportional basis, while a third would be decided in a first-past-the-post vote on specific candidates.

Coalitions would need to get 10 percent of the national vote to get into parliament, while parties running alone would need only 3 percent. Candidates would be chosen by party chiefs.