AS AN INDICATOR of European states of mind, the Common Market's ambivalence toward admitting Spain and Portugal is instructive. The Common Market keeps saying that, yes indeed, it is still committed to expand its membership from 10 to 12. But progress has slowed to an unconfident walk. The negotiations illustrate the two threads in Common Market politics--the original idealism it was founded on and the preoccupation with financing and trade disputes that has increasingly absorbed its energies.

The purpose of the Common Market, as its architects understood it, went far beyond commerce. The idea was to use trade to tie ancient enemies inextricably together in a structure that would provide a stable economic base for liberal democracy. A lot of Europeans to the north felt strongly that when the Iberians finally shed their dictatorships, the Common Market had a positive responsibility to reach out and tie them firmly to democratic institutions. It was an opportunity for international uplift on a grand scale. Incomes in Spain are less than half the French-German level, and incomes in Portugal are less than half Spain's.

It has now been nine years since a coup overturned Portugal's authoritarian regime, and nearly eight years since Francisco Franco died in Madrid. The talks with the Common Market are supposed to bring the new members in by the beginning of 1985, but clearly that target won't be met. Within the market, attention has swung to the practical costs of admitting two more low-wage countries at a time when unemployment in the present 10 member countries is very high and still rising.

Second thoughts about the new members are sharpest in the regions that would have to compete with their agricultural production. Farm politics is no less sensitive in Europe than anywhere else, and the French and Italian governments are uneasily aware that their farmers would bear the brunt of an influx of cheap fruit, vegetables and wine. As the debate wanders along within the Common Market, Iberian pride is beginning to bridle, and some Spaniards have begun to wonder audibly whether they wish to become the objects of such reluctant do-goodery.

It is not one of the more dramatic choices that Western Europeans are going to have to make in the next several years. But it is likely to have as much influence on the character of the Common Market itself as on the course of politics in Portugal and Spain.