The big loser in this week's Spanish national referendum approving the new democratic constitution may have been the idea that Spain can be ruled from the center.

Despite what was undoubtedly the most intensive get-out-the-vote drive in the country's history, a third of the electorate stayed home in the referendum on the kind of government the Spanish people want to establish.

For a referendum on such a basic issue it was a poor showing, Spanish political analysts of all stripes agreed. In addition, 8 percent of the registered voters said "no" or cast blank ballots.

El Pais, the newspaper closest to the Spainsh government, editorialized: "The most feared adversary of the 'yes' vote was not the 'no' vote but abstention. In this regard, the result was a setback for those of us who believe that consolidation of democratic institutions has as its only guarantee the support of the massess."

Close study of the results shows that, aside from the expected disaffection in the separatish-inclined Basque country, the weakest turnouts and the highest nay votes were in the middleclass precincts that had previously been the heart of Premier Adolfo Suarez's ruling Center Democratic Union.

The left was justifiably pleased with its showing. The Socialists and Communitsts brought their troops out to the polls. But top leftists expressed as much private dismay over the middle class' desertion of the Center Democrats as the party's own analysts.

The leaders of the two main leftist parties, Socialist Felipe Gonzalez and Communist Santiago Carrillo, worked hand in glove with Suarez on the constitutional text. This effort across party lines showed that the leaders of the major groups had understood the lesson of the Spanish Civil War of the 1930s -- that the left cannot rule alone against the right without producing an army-led rebellion.

Failure to understand that produced the 40-year dictatorship of Generalissimo efrancisco Franco. Although avowedly a man of the right, Franco finally tried during his last 15 years in pwer to bridge the gap between Spain's highly politicized poor and the ruling class with its institutions of power by creating a modern economy driven by a broad middle class that could become Spain's natural center.

The extreme right, the remnants of those who had resented Franco's encouragement of modernizers like Suarez, weree pleased by the referendum's outcome.

Antonio Izquierdo, editor of the mass-circulation, nationalistic newspaper El Alcazar, said, "Spain cannot be governed in the center because there is no center here. Both Felipe Gonzalez and Suarez are men who were simply invented by political computer machines.They represent no realities. The Spanish people are extremists by nature. This country is governable only when it has strong leadership."

Suarez's attempts to make himself the indispensable man of the Spanish consensus have failed at least momentarily. This puts the spotlight back on the man who chose Suarex for the task, King Juan Carlos, Franco's handpicked successor.

The new constitution reduces the powers the king exercised during the three-year transition period following Franco's death. Suarez's entourage had thought this wa a safe course since Juan Carlos' indispensable role of maintaining the loyalty of the armed forces was thought to be no lonegr essential.

But analysts concerned about the future of democracy are troubled by the vote of the military. The army vote, concentrated in districts with military housing, produced heavy "no" votes.

The opposition to the constitution expressed by the primate of Spain, Cardinal Marcelo Gonzalez Martin of Toledo, is widely cited as playing a large part in the disaffection of the middle class. In Toledo, the old imperial capital, the "no" vote was 19.8 percent.

The disaffection of a large part of the ruling party's voters drastically narrows Suarez's room for maneuver. He is already being pressed to shift rightward to recapture his voters. It seems increasingly unlikely that he will now has been ruling at the head tions. He has been ruling at the head of a minority government, with 164 of the 350 parliamentary deputies.

The general presumption was that Suarez would try to captalize on a referendum triumph by calling elections. Now, such a course almost surely would bring big gains to the Popular Alliance, the rightist party of Manuel Fraga Iribarne, who was Franco's information minister.

Fraga is described as exultant over the results, claiming credit for the high abstention. Reliable sources said he offered to negotiate with Suarez on the formation of a coalition. Along with a few independents, Fraga's 16 votes in parliament would provide Suarez with a small majority.

The Suarez forces feel they no longer can rule as a minority government now that the consensus approach of issue-by-issue coalitions to write the constitution seems to be played out.

Suarez has another alternative, a "grand coalition" to defend domocracy with Spain's second party, the Socialists, led by Gonzalez. But, a Center Democratic Union strategist. pointed out, "that might be our party's greatest possible service to the nation. But it would be our last service. After that, we would probably disappear as a party."

At various times, the polls have shown the Socialists to be the largest party.With the respectability of Cabinet posts for the first time since the civil war of the 1930s, this analyst reasoned, the Socialists could soon sweep in with a parliamentary majority.

Suarez also is expected to tilt to the right to reassure the army. The military reportedly is disillusioned by the Suarez effort to woo the moderate Socialist voters into his camp.

Suarez is said to fear that while his alliance with Fraga might millify the military, it would probably create a Communist-Socialist alliance leading to an electoral victory by a new popular front that could be followed by a military rebellion on the lines of the civil war that brought Franco to power.