THE EUROPEAN Common Market, one of the postwar world's best ideas, seems to have fallen into a somber phase of backsliding and internal quarreling. When the people who run the 10 countries' governments meet in Stuttgart this weekend, they will face an agenda of disputes that, by their long familiarity, contribute to the sense of paralysis. The unemployment rate in the Common Market is now higher than in this country, and, unlike the American rate, it is likely to keep climbing for the next couple of years. The effect is, perversely, to push the 10 countries away from common action rather than toward it.

Now that the Common Market is 25 years old, Europeans tend to take its benefits for granted while complaining about its costs--which, you have to concede, are substantial. Its large budget is mostly agricultural subsidies, of which France and a couple of the small countries have been the chief beneficiaries over the years. The British, who are paying much more and getting back less than they expected, are again pressing demands for basic changes in the formula. Worse, the whole agriculture program is going to run out of money next year, requiring either a tax increase or cuts in subsidies. Everyone can agree it's not a good time to add to the burdens of the French government, already under severe financial strain. But the subsidies keep producing hugely expensive farm surpluses, and the Common Market's desperate attempts to dump them abroad are bringing it into sharp conflict with other exporting countries--most notably the United States.

The Common Market stands for the free flow of goods among the members and the abolition of all internal tariffs. But other, less conspicuous barriers to trade are rising here and there. The Economist reminds its readers occasionally that the price of cars in Britain is about 40 percent higher than on the Continent. Level competition would be the end of British Leyland, and there seems to be a tacit agreement among its competitors not to go that far. Germany maintains a highly complex system of product standards that comfortably shelter some of its industries. When France choked down its imports of videotape recorders last year by requiring them all to move through one small and remote customs station, it was not only the Japanese exporters who were hit but the Germans as well.

Open markets work better than divided and restricted ones. But in adversity, the temptation to keep restricting them, and to keep raising the subsidies, becomes intense. The politicians meeting in Stuttgart are all supporters of the Common Market in principle. It's the practice that has become unexpectedly difficult.