Gunmen today assassinated the military governor of Spain's Basque province of Gulpuzcoa in the latest of a series of murders apparently aimed at scuttling a Basque home-rule referendum.

The death of Lt. Gen. Lorenzo Gonzalez-Valles y Sanchez, shot in the back while walking with his wife along a promenade in the resort town of San Sebastian, marked the third murder of a senior military officer in the Basque region in less than a week.

The Basque separatist organization ETA claimed responsibility for the shootings Wednesday of a colonel and a major in the city of Bilbao.

Today's murder left little doubt that Spain is facing a major campaign by ETA aimed at provoking the conservative military hierarchy into retaliation before a vital home-rule referendum scheduled to be held in the Basque area a month from now.

Hours after the San Sebastian shooting, Prime Minister Adolfo Suarez called an emergency meeting with senior ministers and the chiefs of staff of the three armed services. Since the double murder in Bilbao, several generals have criticized what they view as the government's soft handling of the terrorist issue, and feelings are believed to be running high in military circles.

The ETA campaign against the military has been code-named "Operation September" by Spanish Intelligence services, which leaked the purported terrorist plot to the Spanish press. It allegedly involves attacking military targets and aims to sabotage the home-rule referendum scheduled for October 25.

Home rule was negotiated by the government and the moderate Basque Nationalist Party last July. It grants sweeping powers of self-government to the Basque provinces on such matters as local taxation, police forces, public works and the development of industries. It constitutes a complete break with the existing highly centralized Spanish administration.

The referendum is bitterly opposed by the Basque separatists because it enshrines the basic principle of the unity of Spain and leaves out of the homerule deal the border province of Navarre, which is claimed by the separatists as an integral part of the Basque country.

ETA has issued statements warning it would step up its terror campaign, and ETA political front organizations have ordered their supporters to boycott the referendum.

By launching "Operation September," ETA apparently is striking at the nerve center of the conservative military establishment, which always has believed that the government's plan to decentralize the administration inevitably will lead to the dismemberment of Spain.

In a front-page interview published today by the newspaper ABC, a leading general said the Army should intervene in politics, particularly in the Basque area, if "it became evident that the police and the judiciary were insufficient to deal with terrorism."

Several other senior officers have called for a tougher line against the Basque guerrillas in recent days, arousing concern that the latest murder will sow further discontent in the military's upper ranks.

Thirteen senior officers, including four generals, have been killed in Spain in acts of political violence in less than two years.

A major worry for the government is that the continuing violence in the Basque country, where nearly 100 persons have died in the past 18 months, could reduce the home-rule referendum to a sham of a democratic process. The fear is that many voters will be intimidated into staying away from the polls and that the turnout will be below 50 percent in certain areas.

The shooting of Lt. Gen. Gonzalez-Valles, as he strolled in civilian clothes accompanied by his wife, highlighted the ruthlessness of the area's Basque guerrillas.

Two young men approached him from behind, and one of them shot him at point-blank range in the back of the head. Both then escaped in a waiting car, whose driver had kept the engine running.

Police set up roadblocks, and the nearly French frontier was sealed, but there was no sign of the killers by late evening.

ETA, which seeks to set up a Marxist independent state, enjoys wide support in San Sebastian -- a once fashionable seaside resort that in recent years has become the center of violent demonstrations and terrorist shootouts.