Fearing repercussions at home and abroad that could damage President Reagan's new push for arms control, the Pentagon today launched an all-out effort to convince people that the United States did not think a nuclear war was winnable and was not suggesting that such wars, if they occurred, would have to be "protracted."

In a speech at the Army War College here, Defense Secretary Caspar W. Weinberger said that the United States must be capable of fighting a protracted nuclear war to convince Moscow that American forces could survive and respond to an initial attack and thus deter the Soviets from launching such a strike.

Senior defense officials held a background briefing for correspondents later in the day at the Pentagon to stress Weinberger's point.

"No one is suggesting that a protracted war is a good thing, or something that we would want to do or are planning to do," said the high official, echoing Weinberger's thoughts. But the ability to deal with a protracted attack is important to deterrence, the official claimed, because Soviet nuclear forces may be large enough to strike several times.

The sudden Pentagon concern over this issue stems from recent leaks to the press of secret documents providing guidance to the Defense Department on future budgets. One account of this guidance, published in The New York Times, emphasized what it called this "first strategy" for fighting a protracted nuclear war.

Senior officials today acknowledged that they are upset by subsequent criticism in the Soviet press and by some newspapers in this country on the Pentagon plans, at least as they have been portrayed in some accounts. The official also acknowledged that the criticism could hurt Reagan's arms control initiatives and his current trip to Europe.

The new Weinberger guidance documents, however, do contain language that says U.S. forces and communications must be able to support "controlled nuclear counterattacks over a protracted period . . . . "

The military has been trying to draw up plans for responding to more than one Soviet strike in a variety of ways for many years. Previous documents on the subject were produced during the Nixon and Carter administrations.

The Weinberger document also makes general points that the U.S. military does not yet have many of these capabilities, and thus these new strategies are based upon future weapons and command systems.

In his speech, Weinberger urged support for Reagan's new strategic buildup, including the MX missile, B1 bomber, Trident II submarine missile and new communications systems.

"But nowhere in all of this do we mean to imply that nuclear war is winnable. This notion has no place in our strategy. We see nuclear weapons only as a way of discouraging the Soviets from thinking that they could ever resort to them," Weinberger said.

"That is exactly why we must have a capability for a 'protracted' response--to demonstrate that our strategic forces could survive Soviet strikes over an extended, that is to say, protracted period.

"Our entire strategic program, including the development of a protracted response capability that has been so maligned in the press recently, has been developed with the express intention of assuring that nuclear war will never be fought."