Winding up a tour of European capitals, Spanish Prime Minister Adolfo Suarez declared today that Spain should join Greece and Portugal as new members in a twelve-nation Common Market by 1981.

Speaking to journalists after a two-day visit to European Economic Community headquarters, Suarez said that while enlargly the Common Market poses some economic obstacles, Europe "would be strengthened and its role in the world greatly enhanced" once the three nascent democracies were tied to the EEC axis.

Despite the optimistic tone struck by Suarez and the soothing words of solidarity he heard furing trivs to EEC capitals, most member countries harbor deep misgivings about the consequences of enlarging the community.

All EEC governments claim it is a moral duty to accept Spain, Greece and Portugal as full-fledged members, but they disagree over the manner in which the Common Market should incorporate the three Mediterranean countries.

The potent farm lobbies in France and Italy fear competition from Spanish products and vow they will fight any concessions. Germany is worried about hordes of migrant workers arriving from southern Europe and Aggravating its employment troubles. Belgium and Holland believe drastic reforms of EEC institutions and voting procedures must take place or the community will collapse under the strains of enlargement.

Only Britain has expressed the view that the expansion might have a tonic effect. Prime Minister James Callaghan said recently that it would compel the Common Market to end all talk of eventual unity, a cynical perspective that drew bitter attacks from Germany and the Benelux countries.

During his discussions with European leaders, Suarez sought to dispel the rampant fears that enlargement would weaken the fabric of the community.

"Spain is willing to work for wide-spread reform of European agriculture, as well as help the Common Market improve its decision-making structure," he said at his press conference today. "Spain's membership will, in the end strengthen the European community."

Addressing the problems posed by the EEC policy of free circulation of labor, Suarez said, "Our aim is not to export labor but to keep our workers in Spain." But he added that Spainiards who leave for jobs in other parts of the Common Market "must have the same rights as other workers."

While rejecting the notion of a preparatory phase, in which EEC aid would help prepare the Spanish economy for the stiff competition it would face upon full entry into the Common Market. Suarez acknowledged that a "post-entry transition" would be necessary.

He said he favored an adjustment period "similar to the one Britain has had since it joined the community four years ago," Suarez noted that industry and agriculture would require a transition phase, but that in all other aspects, such as reaching joint foreign policy positions, Spain must plea an equitable role.

Suarez said he expected Spain to sign the membership treaty by 1981.

"Spain's rapid progress toward democracy, supported by all our political parties, proves that we are ready to join the rest of Europe," he asserted.

Turning to the question at possible Spanish entry into NATO, Suarez noted that the Socialist and Communist parties remain staunchly opposed to Spain's affiliation with any military bloc. "It is a question that only the Spanish parliament can decide at some point in the future," he said.

Spain's defense pact with the United States, providing for the maintenance of U.S. bases in Spain, runs until 1981. "Spain makes money from the U.S. deal but would have to pay its own share as a NATO member," a NATO source observed.