Spanish and U.S. officials failed again Friday to reach agreement on a new treaty to keep U.S. military bases in Spain, sending their negotiations into a critical final phase that will determine the bases’ fate.

Both sides sought to minimize their failure and emphasized that they have decided to meet next month for an eighth round of talks on the bases, which grew out of a joint defense agreement signed in 1953, when Gen. Francisco Franco was the chief of state.

But Spanish officials said they will formally notify the United States by letter next week that they do not want the present treaty automatically extended for another year when it lapses next May 14. That, in effect, sets a six-month deadline for the two sides to agree on a new treaty.

If they fail, the United States will have a year to withdraw its 12,500 servicemen and women, its planes, ships and other equipment from the bases.

But an American spokesman who took part in the negotiations said that “neither government wants to see it (the letter) as some kind of rupture.”

Inocencio Arias, the spokesman for the Ministry of Foreign Affairs, seemed to agree, telling reporters that Spain is doing “no more, no less” than invoking a provision of the treaty allowing either side to prevent its automatic extension.

The most contentious issue for both sides is the U.S. Air Force’s 401st Tactical Wing--72 F-16 jet fighters based at Torrejon, just outside Madrid. From the start of negotiations in July, 1986, Spain has demanded that the United States withdraw the planes. Until recently, the United States insisted that it could not do so--that they are a vital element of North Atlantic Treaty Organization defenses.

But in the latest round of talks, which lasted for 2 1/2 hours Thursday and a little more than two hours Friday, the United States, according to the American spokesman, made a formal proposal for a “partial redeployment (of the F-16s), under certain conditions, out of Spain.”

The spokesman, who spoke on the condition of anonymity, hinted that the “partial redeployment” would involve a squadron of 24 planes.

“To come to the point of making an offer to redeploy some of the planes,” he said, “is a very difficult thing for the United States to do.”

The proposal, which had been discussed earlier in the week by Spain’s defense minister, Narcis Serra, and Secretary of Defense Caspar W. Weinberger at a NATO defense ministers meeting in Monterey, Calif., was formally rejected by the Spanish negotiators in Madrid as inadequate. Serra had described the American proposal as “unfortunately, still not enough.”

The negotiations have been complicated by deep and sometimes bitter feelings. For many Spaniards in newly democratic Spain, the issue of the bases is fraught with emotion. The 1953 treaty that authorized the bases was seen by many opponents of Franco as an official American endorsement of his dictatorship. Many Spanish leftists still bristle at the recollection of an old photograph that shows President Dwight D. Eisenhower embracing Franco.

As Defense Minister Serra put it in a speech last September, the treaty, which has been renewed twice, “bears the stigma of its origin.”

“To allow things to remain as they are,” he said, “can only be regarded as an imposition, dating back to circumstances now far behind us.”

Some of these feelings have been reflected in the huge demonstrations that leftists, including officials of the governing Socialist Party, have mounted against the bases in the past few years.

Many Spanish analysts believe that American officials failed to comprehend the depth of this emotion. The Americans, according to this Spanish view, were convinced that Prime Minister Felipe Gonzalez would only go through the motions of trying to fulfill his pledge to Spaniards that he would reduce the American presence in Spain if they would vote in favor of Spanish membership in NATO in a 1986 referendum.

In a sarcastic commentary Friday, the leftist newspaper Diario 16 said that the Reagan Administration had thought the negotiations would be only cosmetic and that former U.S. Ambassador Thomas O. Enders and present Ambassador Reginald Bartholomew had both created false expectations in Washington.

But the U.S. spokesman denied that the Americans had failed to understand the depth of feelings here.

“We understood,” he said, " that the Gonzalez government was dealing with its history.”

Ambassador Bartholomew led the American delegation and Maximo Cajal, secretary general of the Foreign Ministry, led the Spanish delegation in the latest round of negotiations, which the American spokesman described as “relaxed and fluid.”

A possible way out of the impasse came from Lisbon. Eurico de Melo, the Portuguese minister of defense, reversed position and said that Portugal could serve as a base for the 72 F-16s if they are forced to leave Torrejon.

Portugal, which shares the Iberian peninsula with Spain, presumably is close enough to Torrejon to convince American officials that it could carry out the Torrejon mission without jeopardizing NATO defense.

Spain has been subject to a good deal of pressure from NATO allies to refrain from doing anything that might weaken European defense or American involvement in European defense. Sensitive to this, Spanish officials have insisted that the removal of the F-16s from Torrejon would not harm European defenses. Echoing this, Arias, the Spanish spokesman, told reporters Friday, “Spain is not trying to leave Europe.”

American bases in Spain include Torrejon, which has the longest runway in Western Europe; an in-flight refueling and pilot training base in Zaragoza, and the Rota naval base at the mouth of the Mediterranean. The Americans also operate another air base at Moron and nine small installations involved mainly in communications.