When Spain and Portugal join the European Community next January, it will become -- at least potentially -- an economic power equal to the United States. The 12 countries of the community, taken together, will be very close to the American level of economic output. Their population will be a third larger. The community's actual power will depend on the 12 countries' determination to keep pressing toward greater unity. The final agreements on Spanish and Portuguese membership are the latest demonstration of progress there.

The negotiations went on for eight years. Although the European Community is built around a common market, the real motives for founding it and then for expanding it have never been essentially economic. The idea has always been to use economic growth to strengthen the base for stable and vigorous parliamentary democracy. The question was whether that high purpose would fade after the first burst of postwar idealism. That hasn't happened.

Perhaps there are certain commercial advantages to some of the community's 10 current members in bringing in two more. But there are also clear disadvantages to several -- France, Italy and Greece -- whose farmers will now be subjected to fierce competition from Iberia. That's why the negotiations dragged on so long. The reason for their eventual success was the strong interest in other Western European countries in tying Iberia more closely to its democratic neighbors, after the last of the pre-war fascist governments collapsed there in the mid-1970s.

There have been other signs of real vitality in the community over the past decade. It began choosing its European Parliament by direct election in 1979. That year it also established the European Monetary System that ties its currencies -- with the exception of the free-floating British pound and the Greek drachma -- to each other. This monetary system is sometimes dismissed as a mere technical arrangement, but it's much more than that. To link currencies together requires close coordination of national economic policies. The joint monetary system is the most important advance of the European federal principle since the founding of the common market itself.

The mood of politics in Western Europe continues to be somber, oppressed by extremely high unemployment and comparatively slow economic growth. The interesting thing is that in this atmosphere the European Community continues to develop, suggesting that it draws its strength from sources deeper than the passing cycles of prosperity. Jean Monnet, the great Frenchman who was the community's chief architect, would have been gratified, but not surprised.