The leader of the Five Star Movement, Beppe Grillo, delivers a speech on Dec. 2 in Turin, Italy. (Marco Bertorello/AFP/Getty Images)

Italy’s anti-elite parties vowed Monday to join forces with other insurgents around Europe as Prime Minister Matteo Renzi prepared to resign, but it was unclear whether an unlikely alliance of disaffected voters could propel the populists to office.

Italy plunged into political uncertainty Monday after the decisive defeat of Renzi’s signature constitutional revision plan, which was meant to strengthen his hand and defuse the establishment-bashing movement. The rejection by voters had the opposite outcome.

Now, anti-immigrant, anti-euro populists on the left and right who have steadily built power as an alternative to Italy’s old-guard ­political leadership are seeking to transform their success into a chance at their nation’s highest office. The eclectic Five Star movement is polling at 30 percent, neck and neck with Renzi’s center-left Democratic Party.

“A lesson for everyone: You can’t always lie to the people without suffering the consequences,” said Beppe Grillo, the caustic comedian who founded the Five Star movement.

But markets and leaders elsewhere in Europe reacted with caution Monday to the Italian shifts, reflecting the steep barriers that remain before insurgents could capture Italy’s top job.

The youthful, Coca-Cola-
guzzling Renzi had portrayed himself as a lone warrior against the Euroskepticism that fueled Britain’s vote in June to break with the European Union and gave tail winds to right-wing political leaders in France and elsewhere.

“This is something that is going on all over Europe, if you look at the momentum,” said Manlio Di Stefano, a Five Star lawmaker in the lower house of Parliament. “People are really tired of the same kind of politics. Every term is the same, every government is the same, so people are reacting.”

But Renzi also alienated Italian voters over 2½ years in office by failing to jump-start growth in Europe’s fourth-biggest economy. The referendum results — 59 percent of voters opposed the revisions — may have reflected enmity toward Renzi as much as a wave of populism washing over the Italian electorate.

Much like the coalition of ­working-class voters and the right-wing establishment that drove Donald Trump to victory in the United States, the forces that united to defeat Renzi came from across Italy’s political spectrum. The diverse views may make collaboration difficult.

The decision about Renzi’s successor as prime minister falls to Italian President Sergio Mattarella, an establishment stalwart who is unlikely to clear an easy path for Five Star backers. A likely scenario could be a caretaker government headed by a different leader from Renzi’s center-left Democratic Party who would hold office until new elections in early 2018. Mattarella asked Renzi on Monday to hold off on his resignation until the government passes next year’s budget, a potential delay of a few days or a week that will buy some time for succession planning but does not significantly alter the political situation.

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Once Renzi’s successor is announced, the process of forming a new government will take weeks.

Italy is no stranger to political chaos, and the next government will be the 64th in 70 years. But many Italians are growing weary of a long stretch of unelected prime ministers, going back to 2011.

“The people do not recognize their vote in the government. Because it’s what in the E.U. context is called a deficit of democracy,” said Stefano Stefanini, a former senior Italian diplomat who is now a political consultant in Brussels.

Mainstream parties also plan an overhaul of an electoral law that would make it far harder for the Five Star movement to govern without a coalition, yet another barrier to its quest to capture office.

The Five Star movement has a patchwork collection of policies that are more Bernie Sanders than Donald Trump. Grillo, the founder, has praised Trump and agitated against migrants, but many of the movement’s rank and file have softer edges. The party has promised to hold a referendum on Italy’s membership in the euro zone if it captures office, criticizing the currency as a project of the elite that has harmed ordinary Italian workers. The movement also agitates against corruption and for the environment. And it forms many of its policies through an online platform for its members that it brags is a cutting-edge tool for direct democracy.

But when Five Star members have captured power, they have sometimes had difficulty governing, most notably in Rome, where a Five Star mayor has struggled to deliver public services since she took office in the spring.

The mayor of Turin has done better. But she trained at one of Italy’s top universities, is from a prominent business family and has governed as a moderate.

“The Italians are essentially unhappy with the establishment, with the establishment parties. They are looking at the Five Star movement, but they are still looking for credible faces,” said Wolfango Piccoli, co-president of ­Teneo Intelligence, a political risk consulting firm.

Even if the insurgent populists never come to office, they are guiding the national political discussion by sending mainstream politicians scrambling to appeal to their anti-system voters.

“By simply being there, they shape the political agenda, and they force the system to react to them,” Piccoli said.

If the Five Star movement does succeed in calling a referendum on the euro, the move could pressure Italy’s shaky banking sector and rekindle the euro crisis. Shares in Italian bank stocks were broadly down Monday; the nation’s most troubled bank, Banca Monte dei Paschi di Siena, lost 3.9 percent in late-afternoon trading.

But reflecting the overall short-term calm, Italy’s main stock exchange in Milan was slightly down in late-day trading. Many other European markets, including London’s FTSE and Germany’s DAX, were up.

European leaders said Monday that they were confident Italian politicians would overcome the turbulence without setting off a broader crisis on the troubled continent, which has struggled in the face of Greece’s fiscal collapse and an unprecedented surge of migrants and refugees. 

“This is a strong country with strong government, and I have every confidence in Italy to deal with the situation,” Pierre Moscovici, the senior E.U. official in charge of economics, told France 2 television.

Yet the Five Star movement is surging against its competitors. 

In head-to-head polling against the ruling Democratic Party, it is the clear favorite, with an advantage of 53 percent to 47 percent, according to a survey from the EMG polling group released Sunday. Measured against all the parties in Italy, Five Star and the Democratic Party are in a dead heat.

Renzi allies said Monday that the rejection of the constitutional changes has dealt Italy a significant blow.

“The risk of disintegration began with Brexit, when a very important member was lost,” said Sandro Gozi, a top Renzi ally who is Italy’s undersecretary of state for European affairs. “The European Union needs to depart from the status quo, because it makes the future much harder.” 

Birnbaum reported from Brussels.