Washington

ALONGSIDE Greek debt and the Libyan intervention, European Union countries are bickering over another issue, one that could well determine the future of their would-be megastate: immigration and internal borders. A growing number, including Italy, France and Denmark, want to carve out exceptions to the agreements under which member states open their borders to one another.

The issue has been simmering for years, but unrest in the Middle East and North Africa and fears of a new wave of migrants have brought it to a boil. Of course, closing off Europe to newcomers violates the cosmopolitan vision on which the European Union was built, and doing so could kill the project altogether. But as the continent’s leaders are now learning, it’s also possible to kill Europe by opening its doors wider than its citizens will tolerate.

The present crisis started when refugees began fleeing Tunisia by boat in the wake of January’s revolution. Italy was a natural destination: its island of Lampedusa lies south of Tunis and just 70 miles off the African coast. These refugees were joined by others from Libya, and by late May almost 40,000 had arrived.

Under ordinary circumstances, if you make it to Lampedusa, you can get to Paris or Berlin with few questions asked. The Schengen agreements, signed in 1985 and 1990, permit passport-free travel within 22 continental countries of the European Union (Britain and Ireland are among the exceptions), as well as non-Union signatories.

Along with the euro, Schengen is Europe’s symbol, a milestone in its integration — on a continent long hemmed in by nationalism and bureaucracy, an Italian can travel to Paris without showing papers or changing money. And it’s growing: the European Parliament this week voted overwhelmingly to recommend extending Schengen to the European Union’s two newest members, Bulgaria and Romania.

The union’s treaties assume that whatever country receives migrants will also process their asylum applications and look after the migrants during the adjudication of their status. But Italian bureaucrats are overwhelmed by the tens of thousands of applicants, and the Italian public, like most of the rest of Europe, does not like mass migration.

With the slyness that is his political calling card, Silvio Berlusconi, the Italian prime minister, issued six-month residency permits to 8,000 of the newcomers this spring, allowing them free movement within the European Union. The Tunisians, who are largely Francophone, then headed for France, turning Mr. Berlusconi’s political problem into that of the French president, Nicolas Sarkozy.

In April Mr. Sarkozy shocked European leaders by re-establishing Franco-Italian border controls for several hours. Under Schengen, countries may re-man their borders only if there is a “grave threat to public order or internal security” — say, soccer hooligans. But Mr. Sarkozy and Mr. Berlusconi recently wrote a joint letter that called for putting the agreements on hold in case of a big refugee influx.

Any time Europe’s external borders look porous, individual countries get nervous. Denmark’s prime minister, Lars Lokke Rasmussen, has called for refortifying the country’s borders with Germany and Sweden, prompting accusations that he means to undo Schengen. It’s not all that surprising; Denmark is the Arizona of Europe — the European country most hostile to mass immigration — and Mr. Rasmussen is behaving as Arizona Gov. Jan Brewer did last spring, taking federal enforcement responsibilities into local hands.

Such steps are signs of an even broader movement. Populist parties, anti-immigrant and anti-elite, are rising in many European countries. The National Front in France, the Freedom Party in the Netherlands and the True Finns have all polled at 20 percent or more in recent elections. After years on the fringe, they have become mass parties, putting the heat on center-right governments like those of Mr. Berlusconi and Mr. Sarkozy.

Establishment politicians look askance at the public’s concerns about Schengen; Germany’s foreign minister, Guido Westerwelle, has called it “such a success that it should not be renegotiated.”

Things look different from the populist point of view. Under the current system, individual countries are responsible for policing their share of the continent’s external borders, which is great for, say, Luxembourg but unfair to countries on the union’s southern edge. It is also an incentive to abuse: migrants can enter through the most lenient port of entry and then, thanks to Schengen, move toward the country most welcoming to immigrants or most generous with welfare benefits.

What to do about it is a harder question. Since Schengen is about convenience for Europeans, stopping travelers at the old national borders would defeat its purpose altogether. Limiting enforcement to those who “look foreign” may not survive contact with human-rights litigators, as the experience with Arizona state law and airport security has shown.

The only alternative, if Europe wants to keep Schengen intact, is to beef up the woefully underfunded European border patrol, Frontex. With a budget of only 88 million euros (about $128 million), it is cobbled together out of various nations’ unneeded naval vessels and military personnel.

The European Commission received the Berlusconi-Sarkozy complaint in a bureaucratic spirit, expressing a desire to “replace the unilateral re-introduction of border controls by a community mechanism.” This is the commission’s response to everything: to urge that the union’s prerogatives be further consolidated. As the euro has faltered, for instance, the commission has urged more European control of countries’ fiscal policies. In an American context, this would be like responding to the Tea Party’s complaints about big government by promising to create a vast, deficit-funded federal bureaucracy to deal with them. It is a dangerous heaping up of political tinder.

“We want Schengen to live,” Mr. Sarkozy said recently, “but for Schengen to live, it must be reformed.” He has the right idea of what is at stake. Until recently, “building Europe” was easy. European citizens, after grumbling a bit, would reconcile themselves to the plans of pan-European visionaries. They are getting less easygoing by the day. Building Europe now depends on taking an occasional step back, even if that means reassuring member states of their right, in an emergency, to keep watch over their own borders.