Spain's passage from the Franco dictatorship to Western-style parliamentary democracy enters its final phase Wednesday with a national referendum on a new constitution reflecting delicate compromise among the country's major political parties.

The transition from 40 years of authoritarianism to the kind of open society in which most Westerners live will still have a long way to go, however, even after the widely expected favorable vote. The constitution introduces basic liberties but leaves unsettled many of the fundamental questions still dividing Spanish society.

In final appeals to the nation, Premier Adolfo Suarez and Socialist leader Felipe Gonzalez took pains to deny a long list of problems that the extreme right argues would spring from the new constitutional arrangement: divorce, abortion, atheism and particularly a breakup of Spanish national unity.

The conservative fears have been encouraged by a struggle against the proposed constitution waged by Basque separatists in the ETA terrorist group who have called on the Basque population to boycott the referendum. ETA, initials for Basque Country and Liberty, rejects as insufficient the Spanish government's promise of regional autonomy contained in the 169-article draft constitution.

ETA has killed 20 policemen since Oct. 1 to dramatize its separatist campaign. During the same period, it has claimed responsibility for assassinations of 10 other persons, including a judge and a navy captain.

The latest Basque strike came today when a terror squad machine-gunned a cafe in the major Basque city of San Sebastian, killing three policemen.

Despite the election eve violence, a heavy vote for the constitution is treated by everyone here as a foregone conclusion. The only question people are asking is how massive the "yes" vote will turn out to be.

Jesus Sancho Rof, the deputy interior minister in charge of the government's $8 million saturation campaign for the constitution, says the last of his ministry's almost daily polls pointed to a turnout of 75 to 80 percent, with nays from only 9 percent of those voting.

There are those, even inside the Cabinet, who question the effectiveness of the campaign, waged by 12 major advertising companies under government contract. During prime evening time, long advertising spots have been aired on state television every half hour for weeks. Even Sancho Rof conceded that spot checks show "the intellectuals and the politically sophisticated have gotten bored" by the official campaign.

Advocates of a "no" vote have been kept off the air on grounds that they are practically not represented in parliament.

The constitutional text guarantees fundamental political liberties, but leaves deliberately unsettled basic questions raised by the right about what kind of society post-Franco Spain will be. In a typical compromise among the seven parliamentarians who drafted the text, it says in one article that Spain will have a free market economy, and in another that the state may plan the economy.

This approach of leaving basic conflicts between parties of the left and the right unresolved is the heart of the politics of consensus worked out among Suarez's ruling Center Democratic Union, the Communists, Socialists and the "civilized right" of the Popular Alliance.

"The constitution is not the end; it is the start of everything," said Juan Luis Cebrian, editor of E1 Pais, the newspaper the most identified with the consensus approach. "There are more than 50 basic laws that need to be passed."

Almost 25 million people in the population of 36 million are eligible to vote. Eighteen to 21-year-olds vote for the first time.

Copies of the text were distributed to all households. They were printed in six languages and dialects of Spain -- Catalan, Basque, Castillian, Galician, Valencian and Majorcan. Illustrating the problems of regionalism, there was even a debate over whether the constitution could say that Spanish is the language of Spain. Dialect speakers insisted that their tongues were as Spanish as Castillian, which foreigners generally call Spanish. It was finally decided that the official language should be called Castillian, not Spanish.

Sancho Rof concedes that a 50 per cent turnout in the Basque country is the best the government can expect. In the great industrial cities of the region, Bilbao and San Sebastian, it is relatively safe for people to go to the polls, but not in the smaller towns.

Elsewhere, almost daily front-page pictures of bodies and bomb wreckage from the Basque country have fed the determination of the extreme right to vote against what it calls a "Godless" constitution for a "permissive society."

The broadest publicity has been given to the call for a no vote by the conservative primate of Spain, Cardinal Marcelo Gonzalez Martin of Toledo, because the constitution does not mention God. Only eight of the 70 other Spanish bishops joined Gonzalez in his call. It was in apparent contradiction with an earlier statement by all the bishops that Spaniards were free to vote their consciences.

Gonzalez ignored the provision in the constitution that says that, while there will be no state religion, "the government will take into account the religious beliefs of Spanish society and will maintain the corresponding relations of cooperation with the Catholic church and the other confessions."

"Godlessness" is the key accusation of all rightist propaganda. Even though the constitution seems bound to get a ringing electroal endorsement, people like editor Cebrian express fear that it may be gradually sapped of its legitimacy as its opponents fight a point-by-point rear-guard action on every enabling measure.

What worries independent observers most is the coming together of religious opposition with hostility inside the military. For the army and the highly militarized police forces, the touchstone will undoubtedly be how far the government goes in granting real autonomy to the Basques and Catalans.

The constitutional definition of the role of the armed forces almost reads like an authorization to crush any real independence movement.

This apparently has not been reassuring enough for many officers. It is becoming increasingly clear that dozens of high-ranking officers had to have been aware of an abortive plot broken up three weeks ago to take over the prime minister's residence, the Moncloa Palace, and arrest Suarez.

Reliable sources insist that Spanish military intelligence was aware of the plot, but only informed Suarez of it after he made an official inquiry generated by word he got from a loyal police general. Only two officers have been arrested and the government has been strangely silent about the affair since it first broke into the open.

At about the time the plot was discovered, the defense minister was touring major military bases to sell the constitution to the officer corps. When he got to the southern port of Cartagena, the minister, Lt. Gen. Manual Gutierrez Mellado, was met with insubordination.

A naval captain made a speech to him blaming democracy for Basque terrorism. When the minister ordered the captain to stop his harangue, the lieutenant general commanding the Civil Guard police for the region got up and called the Cabinet member a traitor for supporting a constitution dedicated to atheism, Marxism and separatism. The minister had the general arrested, but not before he had received hesitant applause from his fellow senior officers.