SAINT-MARTIN-DE-LA-PORTE, France (Reuters) - Italy’s 5-Star Movement and League party were rivals going into last year’s national election, became coalition partners in a populist government afterwards, and now an Alpine rail tunnel threatens to tear them apart.

The dispute over whether to forge ahead with one of Europe’s most ambitious engineering projects is rooted in the divergent nature of the two anti-establishment parties and highlights their conflicting visions for Italy’s future.

The hard-right, pro-business League wants the tunnel to go ahead, as do two-thirds of Italians, according to an EMG Acqua poll last week. On the other hand, more than half of 5-Star’s left-leaning and environmentally friendly supporters oppose it.

“It’s striking because they are two populist forces with two very different groups of people supporting them,” said Lorenzo Pregliasco, a founder of YouTrend, a political consultancy based in Turin, the northwestern city where trains are supposed to arrive from France after passing through the tunnel.

A 2,400-tonne boring machine nicknamed “Federica” is already working around the clock more than 1,000 meters (3,000 feet) below ground on the French side of the border even though Rome is threatening to pull the plug as it reviews the project.

The tunnel is stoking tension within the coalition because of regional and European elections in May, which could reverse the balance of power within the eight-month-old coalition and destabilize a government that manages one of the world’s largest debts, which is closely monitored by financial markets.

Polls foresee the League surging above 5-Star, which has already compromised on other major battles in recent months but appears not to want to give ground on the 8.6-billion-euro ($10 billion) tunnel.

Transport Minister and 5-Star member Danilo Toninelli told Reuters a decision would come before the elections, and 5-Star bigwig Alessandro Di Battista said the project was “idiocy”.

In December, Italy agreed with France to freeze new contracts on the 36-mile (58 km) tunnel until completion of an Italian cost-benefit analysis by a panel of experts, which Toninelli has said should be published by the end of the month.

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“If the analysis says that we’re throwing away money, only a madman would move forward,” 5-Star leader Luigi Di Maio said on Monday. However, it might not prove so easy to kill the project.

Longest railway tunnels: tmsnrt.rs/2AXyWeb

FEDERICA

Deep in the bowels of the Mount Cenis (Moncenisio), geothermal energy heats the air to about 29 degrees (84 Fahrenheit). With a force equal to the power of eight Formula 1 engines, boring machine Federica rumbles and shakes as she cuts into coal-colored rock, advancing as much as 18 meters per day.

So far the joint French and Italian company - the Tunnel Euralpin Lyon Turin (TELT) - has dug more than 15 miles (25 km) inside the mountain, including four access tunnels and about 4 miles (6 km) of one of the two main passageways.

Some 800 people are employed, either directly or indirectly, TELT says, and that number will balloon to 8,000 if and when work reaches full capacity.

Those in favor of the project, known in Italy as the TAV, say the costs of shutting it down with 25 km (15 miles) already dug could equal those of finishing it.

“You can’t turn off the boring machine, leave it there and walk away,” said Piergiuseppe Gilli, TELT’s chief of construction, who has worked on the project for 22 years.

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The European Union is picking up 40 percent of the cost, which could increase to 50 percent, because it is part of a “Mediterranean Corridor” meant to link rail yards and ports from Spain to the Ukrainian border with Hungary.

French Transport Minister Elizabeth Borne has emphasized the “geostrategic” importance of the project, saying it must not lose EU funding.

Deputy prime minister Matteo Salvini, who is also leader of the League, expressed his categorical support for it on Friday: “The TAV absolutely must be done. It would cost more not to do it than to do it.”

Supporters of the Franco-Italian rail link have staged two large rallies in recent months in Turin, which is about 40 miles (65 km) from the French border. Local League politicians attended the demonstrations.

“It would improve Italy’s links to other European countries and permit faster movement of passengers and goods,” Turin resident Salvatore Bellomo said in the city’s central square. “This project would bring Italy into the future.”

In the past the “No-TAV Movement”, which is also supported by far-left activists, has frequently clashed with police, and after death threats, two of TELT’s top managers are under 24-hour armed guard.

The argument 5-Star makes against the project is that it is a huge waste of public money and a potential health concern for people who live near the Italian work site, where some of the rock dust could release asbestos into the air.

The money would be better spent on Italy’s existing infrastructure, which is crumbling, 5-Star says, citing the collapse of a motorway bridge in August that killed 43 people.

“The flow of goods is not expected to be large enough to justify public works of these dimensions,” said Angelo Tartaglia, a retired professor at Turin’s Polytechic University. “It would not be profitable, and what’s worse is ... the maintenance costs would be higher than the tunnel’s revenue.”

Tartaglia is on a committee of experts formed by the Turin mayor Chiara Appendino after she became one of the first 5-Star heads of a major Italian city in 2016.

The 5-Star fight against the tunnel dates back to a time when it was still trying to make its name known, and the party’s founder, comic Beppe Grillo, was calling for an environmentally friendly economic model based on “happy negative growth”.

But being in government has forced 5-Star to shed some of its anti-establishment identity. The movement has already relented on promises to close the Ilva steel plant or shut down construction of a gas pipeline from Albania.

“These kind of reality checks are part of the nature of this government,” consultant Pregliasco said, referring to Ilva and the pipeline. “The difference with the tunnel is its higher symbolic value for 5-Star.”