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.