FLORENCE — Before there was Brexit or Donald Trump, there was Italy’s 5Star Movement.

Ever since it burst onto the scene in 2013, at the height of the euro crisis, the anti-establishment, Euroskeptic political party has served as the country’s largest, and loudest, opposition group, stubbornly defying predictions of a collapse in support. Recent polls put it as Italy’s second most popular party, close behind Prime Minister Matteo Renzi’s Democratic Party.

The party’s enduring popularity has many in the European political establishment worried about the Continent’s future. And possibly for good reason. The 5Star Movement is calling for a referendum on Italy’s membership in the eurozone and the renegotiation of the country’s staggering public debt — moves that could roil the global economy and reignite the euro crisis, likely leading to the dissolution of the single currency zone and perhaps the European Union.

Whatever the outcome of Renzi’s constitutional referendum on December 4, one thing is clear: The 5Star Movement has never looked as close to power as it does today.

“I don’t understand why the Europe Union isn’t focusing on the reason it’s collapsing instead of worrying about a movement that, when it takes power, will ask citizens for their opinion on the single currency” — Alessandro Di Battista

Should Italians reject Renzi’s reforms, as polls suggest they will, the 5Star Movement will rack up a major victory ahead of elections in 2017 or 2018. Should the referendum pass, a new electoral law will come into effect favoring parties like the 5Star Movement that are reluctant to enter into power-sharing agreements. Securing a plurality of the vote would be enough to propel it into government — a reality seemingly acknowledged by Renzi, who has supported proposals that would modify the law to prevent such an occurrence.

The 5Star “is good at criticizing, but being constructive is another thing,” says Giacomo Vaciago, a former government adviser and an economics professor at Milan Catholic University. “If they were in government, it would be chaos.”

Referendum on the euro

When Alessandro Di Battista steps into Florence’s central square, he’s welcomed as a star. One elderly lady asks him why he’s not an actor. Tall, handsome and outspoken, the 38-year-old Italian parliamentarian is one of the 5Star Movement’s three most prominent figures.

The others are Luigi Di Maio, the 30-year-old vice president of the Italian parliament, and Beppe Grillo, an angry standup comedian who, before he founded the movement in 2009, was best known for holding “Go f--k yourself” rallies aimed at corruption in the Italian political establishment. Should the 5star Movement come to power, many in the party expect Di Battista to become the foreign minister.

Italian governments form and fall, usually with little impact on the country’s policies. After two-and-a-half-years in office, Renzi has already lasted more than twice as long as the average post-war prime minister. But a victory by the 5Star Movement would likely represent a sharp break with the past.

Speaking in front of a replica of Michelangelo’s David on one side of the square, Di Battista is unapologetic about the party’s desire to upset the existing European order. “I don’t understand why the Europe Union isn’t focusing on the reason it’s collapsing instead of worrying about a movement that, when it takes power, will ask citizens for their opinion on the single currency,” he says.

Holding referendums on international treaties, such as the one governing Italy’s membership in the eurozone, is currently banned by the Italian constitution. 5Star says it would change the law to accommodate its initiative. “Personally, I would vote for Italy exiting the monetary union,” adds Di Battista, who spent the summer touring the country on a scooter urging Italians to vote “No” in Renzi’s referendum. “A country that cannot control its own currency cannot implement independent fiscal and monetary policies.”

Polls indicate Italians would not vote to leave the eurozone, but even the prospect of a vote would likely cause markets to panic, putting pressure on the weaker economies of the eurozone. Also worrying the markets are the party’s proposals to renegotiate Italy’s debt burden, which at 132.5 percent of GDP is the second highest in the eurozone after Greece. “We want to sit at a table and talk about the debt, but calmly,” says Di Battista. “This has already been done in many countries in the recent years. It has been done in some Latin American countries.”

Change at the top

Until recently, the possibility of a 5Star government was all but inconceivable. After the political upheaval in the United Kingdom and the United States, many in Italy — and across the Continent — are taking a closer look at how the party operates.

The 5Star Movement doesn’t fit neatly into the traditional left-right division in Italian politics. The fight against corruption is its banner issue, with every headline of bribe-taking or fraud contributing to a bump in the polls. The party’s key initiatives — the “five stars” from which it takes its name — are protecting the environment, ensuring that common goods like water stay in public hands, making internet access a fundamental right and promoting sustainable development and sustainable transport. Anti-globalization is another important theme, and many in the party — Grillo in particular — have also taken a hard line against immigration.

The party fashions itself as a product of direct democracy, without a formal leadership and with important decisions determined by votes on its internet platform. The party’s inner workings have been far from transparent.

Before the 2013 election, the party was largely run by Grillo and the editor of his blog, Gianroberto Casaleggio, a tech guru who died after he suffered a stroke in April. For most of the past three years, Grillo has taken a back seat, allowing leaders like Di Battista and Di Maio to represent the party. The two men were part of a five-member leadership committee — until September, when Grillo took back the reins after reports of clashes within the party.

Di Maio “was thinking to be a primus inter pares,” says Marco Canestrari, a former close associate of Grillo who is now one of the party’s most prominent critics. “But then when the others realized that he was acting on his own, organizing meetings on his own, following his own agenda, the others started waging war against him.”

Open to Moscow

Critics of the party accuse Grillo of ignoring the membership on many important decisions. This year, for example, Grillo brushed off a vote by the party membership in favor of supporting civil unions for gay couples, telling lawmakers that they were free to vote their conscience.

The referendum on the euro is not in the official party program. Neither is the party’s stance toward Russia. Initially, some members of the movement expressed fears that Russian President Vladimir Putin would censor the internet. But after the Russian invasion of Crimea, Grillo softened the party’s tone, noting that Viktor Yanukovich, the Kremlin-friendly president who fled Ukraine after a popular uprising, had been democratically elected.

Last week, at the European Parliament, where the party’s representatives sit in the same group as the United Kingdom Independence Party, 5Star MEPs voted against a motion urging action against Russian “hostile propaganda.”

“We are against a vision that says the Russia is evil and the U.S. is good,” says Vito Crimi, a 5Star lawmaker in Rome.

The party’s Russia policy is evidence, says Nicola Biondo, a critic of the party who once headed their communication strategy in the lower chamber, that the real power lies with a small group of people. “Who decided to support Putin?” he says, noting that the party’s official program doesn’t say a single word on foreign policy. That absence, he adds, is evidence that foreign affairs is a subject that “cannot be touched because it is decided in secret rooms.”

This fuzziness on policy makes it difficult to predict how the party would govern should it come to power. “They have not created a real leadership,” says Federico Pizzarotti, who was elected mayor of Parma as a member of the party, but has since left it. “There are no experts involved, and they don’t have a structure to govern should they take victory into their hands.”

As much as Italian media might speculate on who would lead a 5Star government, Di Battista insists that the movement does not yet have a plan on possible ministers. “We have not thought about it,” he says. “We’ve been focused on our program. But when elections are called, we will present our governing team ahead of the contest.”