President Carter's much-publicized new nuclear targeting guidelines are designed to underscore for Moscow that any kind of nuclear attack on the United States or its allies would be a losing proposition. Defense Secretary Harold Brown said yesterday in the administration's first speech on the subject.

Presidential Directive 59, signed by Carter on July 25, "is not a new strategic doctrine, not a radical departure from U.S. strategic policy over the past decade or so," said Brown in a speech at the Naval War College in Newport, R.I.

Carter's targeting guidelines are basically a commitment to deploy a more versatile, less vulnerable nuclear offensive force -- one that could wage a big or little war if Moscow chose to start one. Anything from Soviet cities to a lone missile complex could be hit under the more flexible targeting.

Brown denied that the president was thereby taking the nation into a new twilight zone nuclear policy, as some critics have contended.

All Carter did, Brown insisted, was codify "the same essential strategic doctrine" in a way that takes into account "current conditions and current capabilities," including Soviet perceptions about winning a nuclear war and U.S. weaponry capable of denying that option to the Kremlin.

At a separate Pentagon briefing on the doctrine, a defense official who could not be identified under the ground rules acknowledged that there was an argument within the administration about issuing such a directive at all.

He further conceded election-year politics helped weight the case for issuing Presidential Directive 59.

The Carter administration has made a number of defense policy decisions in recent months, many of them reversals of past positions, which puts the president in a stronger position to combat Republican standard-bearer Ronald Reagan on the national security issue.

Brown, who is taking a leading role in defending Carter's military policies in this election campaign, yesterday tried to assure critics of the "refined" nuclear policy that keeping Moscow's finger off the button was still the central objective; that the administration was not threatening to attack the Soviet Union first.

But rather than let Moscow believe the United States was muscle-bound by an all-or-nothing-at-all nuclear policy that would force an American president to tolerate some kind of limited nuclear attack by the Soviet Union, Brown said, Carter was serving notice that our arsenal could combat Russia's in kind, if necessary.

"Deterrance must retrain a far wider range of threats than just massive attacks on U.S. cities," said Brown. "Our strategic forces also must deter nuclear attacks on smaller sets of targets in the United States or on U.S. military forces, and be a wall against nuclear coercion of, or attack on, our friends and allies . . .

"We must have forces, contingency plans and command and control capabilities that will convince the Soviet leadership that no war and no course of aggression by them that led to use of nuclear weapons -- on any scale of attack and at any stage of conflict -- could lead to victory, however they may define victory," said Brown.

This requires a "countervailing strategy" that puts more stress on being able to employ strategic nuclear forces selectively, as well as by all-out retaliation in response to massive attacks on the United States."

"It is our policy," continued Brown "and we have increasingly the means and the detailed plans to carry out this policy, to ensure that the Soviet leadership knows that if they chose some intermediate level of aggression, we could by selective, large (but still less than maximum) nuclear attacks exact an unacceptably high price in the things the Soviets appear to value most: political and military control, military force both nuclear and conventional and the industrial capability to sustain a war."

Besides having weapons accurate and powerful enough to blow up Soviet leaders hiding in underground command posts and destroying factories, the United States arsenal still could blow up cities, Brown said.

In short, the defense secretary said, the United States is deploying a versatile nuclear offense and issuing secret targeting instructions for carrying out the various options.

"This is not a first-strike strategy," Brown said. "We are talking about what we could and, depending on the nature of a Soviet attack, would do in response to a Soviet attack.

"Nothing in the policy contemplates that nuclear war can be a deliberate instrument of achieving our national security goals, because it cannot be. . . .

"In delcaring our ability and our intention to prevent Soviet victory" over the full spectrum of nuclear warfare, Brown said, "we have no illusions about what a nuclear war would mean for mankind. It would be an unimaginable catastrophe."

The defense secretary, in advancing the timetable made in previous statements, said the Soviets may already have warheads accurate enough to blow up U.S. Minutemen missiles in underground silos.

"That potential has been realized or close to it," said Brown in his prepared text.

The growing vulnerability of stationary land missiles is at the heart of Carter's case for building an MX missile that moves from shelter to shelter covertly so Soviet gunners could never be sure of knocking it out of action.

"The increase in Soviet strategic capability over the past decade," said Brown, "and our concern that the Soviets may not believe that nuclear war is unwindable, dictate a U.S. need for more, and more selective, retaliatory options."

The Federation of American Scientists issued a statement yesterday terming the Carter nuclear doctrine "a step backwards" and asked for a declassified version of Presidential Directive 59.

The group's executive committee said the emphasis on the ability to knock out Soviet missiles with MX warheads together with the announced warning that Soviet command centers may be attacked encourages the Soviet Union to take a "hair-trigger" approach to nuclear War. "Nothing could be more subversive of our security," said the committee.