Mini-demonstrators for Brexit are seen in front of a miniature of British Parliament in the Mini-Europe miniature park in Brussels on June 20. (Olivier Hoslet/European Pressphoto Agency)

Days ahead of Thursday’s British referendum on whether to break free from the European Union, many here fear the decision could lead to the destruction of one of the most ambitious political projects since the Holy Roman Empire.

Euroskeptics across the continent are salivating at the prospect of Britain’s departure, hoping to sever their own territories from a map that stretches from the sunny coasts of Portugal to the frigid taiga of Finland. With populist parties surging across the continent, the Brits could be only the first to leave.

The region has been dramatically tested in recent years, by the Greek debt crisis, renewed Russian aggression and, more recently, a historic migration crisis. Britain’s exit, officials and experts say, could provide the biggest challenge yet.

Britain’s departure could also damage the union’s relationship with Washington. Britain remains one of the biggest advocates of globalization in Europe, and its exit could give new voice to trade protectionists across the region. Among the casualties, critics fear, could be plans for a massive free-trade deal between the United States and Europe. 

On June 23, Britain faces a fateful decision: whether or not to leave the European Union. And the world will be watching. (Daron Taylor,Jason Aldag,Danielle Kunitz/The Washington Post)

The question is whether a British exit — or Brexit — could spark a stampede for the door in other countries where the Brussels-seated body remains highly unpopular. At the moment, the list of countries that might consider bolting is relatively short: France, Denmark, the Netherlands and a handful of others. But that could change quickly, experts warn. 

Even if nations defuse their own burgeoning Euroskeptic movements, the days in which leaders convened in Brussels to hand ever more sovereignty to the E.U. may be over if Britain departs, diplomats say. That would be a major blow to a project that started after World War II to bind nations together so tightly that they could never battle one another. 

Some of those who have occupied the E.U.’s highest offices now say they were mistaken to think that if they knocked down economic barriers, a feeling of political unity underneath the blue-and-gold E.U. flag would follow. 

“We have the flag and the anthem. We don’t have much of what supports the flag and the anthem,” said Pascal Lamy, who was the chief of staff to Jacques Delors, the leader of the European Commission from 1985 to 1995 and considered one of the main architects of the modern E.U. 

The turmoil “is extremely disappointing to the founding fathers, who thought they were like medieval alchemists,” Lamy said. “They thought they could transform the stone of economic integration into the gold of political integration.”

As “leave” started to beat “remain” in British opinion polls in recent weeks, E.U. diplomats say that their sense of complacency was replaced by deep nervousness. Already, pressure to hold E.U. referendums is leaping across the English Channel. An Ipsos Mori poll last month found that 55 percent of French voters and 58 percent of Italian voters wanted plebiscites of their own. 

In France, where the far-right, Euroskeptic National Front has surged in the polls ahead of 2017 presidential elections, one center-right presidential hopeful suggested holding a referendum as a way of reaffirming France’s commitment to European values. Bruno Le Maire, who was France’s agriculture minister during the presidency of Nicolas Sarkozy, last month proposed giving “the floor back to the sovereign people about the European question.” 

But many analysts are not confident that French voters would choose to remain in the E.U. if presented with the choice. French views of the E.U. are even worse than Britons’, according to a poll this month from the Pew Research Center. The center found that 61 percent of French people have negative views of the E.U., compared with 48 percent in Britain. 

Growing anger at the E.U. is also a product of what its critics see as its many failings: Running Greece into the ground in exchange for the right to remain in the euro zone. Failing to solve massive unemployment across southern Europe. Miscalculating the Russian response to the E.U. bid to bring Ukraine closer to Brussels. And the E.U.’s fractured handling of the refugee crisis — including forging a deal with a man increasingly seen as an emerging dictator at Europe’s doorstep, Turkish President Recep Tayyip Erdogan. Proponents of the E.U., meanwhile, have sometimes struggled to mount arguments that appeal to the heart, not just the pocketbook. Part of the difficulty is that the bloc is now so large and diverse that there is no single, unifying selling point. 

“There is no European ideal that is clearly defined and on which all members would agree,” said Latvia’s ambassador to the E.U., Sanita Pavluta-Deslandes. Latvia joined the E.U. in 2004, in part as a shield from its former rulers in the Kremlin. 

In Vienna on Friday, leaders of right-wing and nationalist parties across Europe gathered for a show of force ahead of the British vote, calling it a step toward what Marine Le Pen, leader of France’s National Front, described as “Europe a la carte.” 

Marcus Pretzell, a German member of the European Par­liament from the anti-E.U., ­anti-migrant Alternative for Germany party, told reporters at the gathering that “if Great Britain should leave the European Union . . . one will perhaps see that, contrary to all prophecies of doom, there’s life after the European Union.” Pretzell’s party recently made sweeping gains in local elections. 

Many European leaders say they will impose tough divorce terms on Britain if it votes to leave, in part because they fear that making it too easy would embolden their own Euroskeptic voices. 

“In is in, out is out,” German Finance Minister Wolfgang Schäuble said in a recent interview in Der Spiegel. He seemed to rule out the possibility that Britain could retain tariff-free access to E.U. nations if it decided to leave the bloc, contradicting a key argument of Britain’s E.U. opponents. 

Still, even if Britain opts to leave, Germany and the rest of the E.U. have massive incentives to find a way to maintain a close economic relationship with London, and vice versa. In a recent op-ed piece in the Euro am Sonntag weekly, Matthias Wissmann, president of the German Association of the Automotive Industry, noted that almost a third of all new cars sold in Britain — or 810,000 — were manufactured in Germany. And of the almost 1.6 million cars manufactured in Britain in 2015, 1.2 million were exported. 

For the thousands of British citizens in Brussels who have devoted their careers to the E.U., the razor-thin poll numbers are a painful counterweight to the influence they feel they have had since Britain joined the bloc’s predecessor in 1973. 

The E.U. “has become far less Francophone, far more Anglophone, far more globalized,” said Michael Leigh, a British citizen who left the European Commission in 2011 as director general for enlargement, one of the highest civil-service positions in the system. 

Now Leigh is a senior fellow at the German Marshall Fund of the United States, watching the turmoil from outside the system.  

“If you actually look at policy and substance, a great deal of what the E.U. does today bears the mark of the British,” he said.

James McAuley in Paris and Stephanie Kirchner in Berlin contributed to this report.