Not too long ago most Spanish democrats fully supported Basque demands for self-rule and campaigned strenously for the release of Basque separatist guerrillas jailed for fighting against the Government of the late Generalissimo Francisco Franco.

Now, however the Basque drive for self-determination and full autonomy has begun to lose backing in Spain.

One of the reasons for the reversal of attitudes is a new wave of Basque separatist violence which has resulted in five deaths in the past week including the killing of a newspaper editor. There have also been bombings, an attempt to steal 150 automatic rifles from army headquarters in the Basque city of San Sebastian, and extortion of large amounts of money from businessmen threatened by basque guerrillas.

Another reason for the change of heart is the rejection by the Basque Nationalist Party and Basque separatist factions of a provision in the draft constitution granting Spain's regions a measure of self-rule.

The new charter, designed to make Spain a modern democracy, must still be approved by Parliament and accepted by the Spanish people in a referendum. Basque nationalists have announced that they will ask their people to vote against it.

The new wave of violence and the Basque Nationalist opposition to the self-rule provision, which has been accepted by Spain's major political parties, have isolated the Basques.

Last week, the Spanish parliament authorized Premier Adolfo Suarez to enact emergency police measures to end "terrorism" in the Basque provinces. Support for such a measure would have been unthinkable only a year ago. But Spanish political leaders openly blame the Basque Nationalist Party for creating a climate of tension which encourages ETA, the separatist underground, to launch terrorist activities.

Right-wing extremists of the anti-Communist Apoltolic Alliance, apparently, Sunday night opened fire on a retired ETA guerrilla leader in the French Basque city of St. Jean de Luez. In the gunfight, Juan Jose Etxable was critically wounded and his wife was killed.

The attacks set off protest demonstrations by hundreds of angry ETA supporters who clashed with riot police last night in Bilbao and San Sebastian, the two largest cities in the restive Basque region. There were new incidents Wednesday when rightists in San Sebastian interrupted the funeral mass for Mrs. Etxable with anti-Basque shouts and gunfire.

"We are fed up with Basque pride and Basque demands," said a congressman from southern Spain. "All of Spain suffered under Franco. Why should they get special treatment? They are no better than we are."

The Catalans have already established a regional government and have accepted the constitution as a starting point. But Premier Suarezis at a loss over how to deal with the Basques. The provisional self-rule government established in the region earlier this year, after bitter negotiations with the Basques, has not worked.

A few government officials, however, think the government and national political leaders do not understand the Basque problem.

"We should have learned that police measures don't work," said a senior Interior Ministry official. Franco tried them for years and he got nowhere. The political compromises made in congress haven't worked. So where do we go from here? To a new Ulster?

The official indicated that the new anti-terrorism measures resulted from a feeling that if the government did not act against Basque separatists, the military would intervene "and put an end to our democratic experiment."

The Communist Party, which has become influential in Spain's decision-making, has joined other Spanish political parties in the anti-Basque sentiment becoming one of the biggest critics of Basque terrorism and Basque nationalist intrasigence. The Communist Labor Union is campaigning against ETA among Basque workers, who are among the best paid in Spain.

But the Communists have little following in the Basque region, where the Roman Catholicism prevails. Many Catholic priests, in fact, help separatist guerrillas.

The middle-class Basque Nationalist Party does not condone violence, but many of its leaders feel that they cannot accept the limited autonomy spelled out in the constitution without losing the support of the people.

Jose Angel Cuerda, a Basque Nationalist congressman, blamed the government of "isolating the region's majority party" and "creating a feeling of frustration which sparks violence among the Basque people."

He said that the party, which still maintains a government in exile dating back to the defeat of the Republic by Franco in the 1936-39 Civil War, is "willing to help in the pacification of the Basque country and the consolidation of democracy, but that at the same time we demand the recognition of the rights of the Basque people."

The rights demanded are nothing less than the nearly independent republic that the Basques enjoyed for a short while during the Civil War. It is clear that neither King Juan Carlos nor Premier Suarez can go that far without a confrontation with the army and the extreme right.

Franco harshly repressed the Basques because he felt they threatened Spain's unity. But the hostility beteen Madrid and the Basque provinces did not prevent the Basques from prospering. Their land became one of Europe's richest industrial regions. Now, their wealth is at stake because many Spanish businessmen refuse to invest there. Multinational corporations, which used to prefer Basque workers and built factories in the Basque country, are looking elsewhere.

To ETA - the small band of separatist guerrillas who venerate the Basque race, its language and its culture - the enemy is Spain, and, to a lesser degree, France. For years ETA enjoyed immunity from prosecution in France when they sought refuge over the Pyrenees in the French Basque Provinces. Indications are now that the French government is beginning to cooperate with Spanish police in tracking down Spanish Basque guerrillas.

Western intelligence agencies keep track of the activities and contacts of the Basque guerrillas. Spokesmen for ETA say that they are in touch with all of the world's "liberation movements" - Palestinians, French Bretons, Corsicans, Africans and Latin American movements.

Basque nationalists contend they can do nothing to deal with ETA until the Madrid government "gives us real democracy and the right to rule ourselves."

The rest of the Spanish people, however, appear tired of the Basque argument.

"We want to start building, not destroying," said a Catalan sociologist. "The Basques want restitution. We want peace and reconciliation. The Civil War is over."

A few government officials, however, think the government and national political leaders do not understand the Basque problem.