If the unthinkable happened and the Soviet Union attacked Washington or some other U.S. target with nuclear weapons, would the president or his successor be able to get the message to crews of American missiles, bombers and submarines to retaliate?

Officially, the White House and Pentagon say "yes" to such doomsday questions. And there is little doubt that, one way or another, there would be retaliation.

"It might take a few days to put together a coordinated zap," said one senior officer closely involved in such matters, "but sometime and somewhere, a small Washington is going to emerge" to give orders to use surviving U.S. weapons.

Yet, this whole supersecret world of exactly how the president would not only communicate with, but exercise seletive control over, U.S. nuclear forces in the chaos that would follow an attack is a matter of grave and increasing concern to many military and civilian defense officials.

The most extraordinary public acknowledgement of this came early in 1980 in a largley overlooked portion of the annual report to Congress by the chairman of the Joint Chiefs of Staff, Air Force Gen. David C. Jones.

Though the United States has several aternative headquarters, some of them underground, for what is called the "National Command Authority," meaning the president, "these facilities would almost certainly be destroyed by a nuclear attack," the report warned.

"The National Emergency Airborne Command Post," a reference to four specially equipped planes kept at Andrews Air Force Base outside Washington where the president could be whisked in an emergency and sent aloft to safety, "may be the only surviving element of the national military command system after nucler attack," the report said.

Jones warned that just a few well-placed atomic explosions high in the air over the United States could cause a "widespread loss of connectivity" in communications.

Such explosions give off great bursts of energy known as electomagnetic pulse (EMP), which could play havoc with radio and telephone communications on the ground and aboard the president's plane, and presumably with U.S. early-warning systems as well.

Though some improvements are being made, the report says that "communications to and from strategic forces are somewhat deficient," and that "uncertainties of assured communications in the periods" during and after an attack still exist.

The situation is also worrisome, though for differnt reasons, to non-governmental arms control organizations who fear if the U.S. command and control lifeline is vulnerable it could weaken deterrence and either tempt the Soviets to strike first in a crisis or make it impossible to turn off a war once it starts.

The Federation of American Scientists, an organization that frequently opposes Pentagon projects, wrote in a recent newsletter that "Nothing ought to be of more concern . . . then the growing disproportion between the extraordinary good ability to command, control and communicate with strategic forces before they are attacked and the very poor ability thereafter."

The United States has an elaborate communications network in place which, according to congressional testimony, includes 43 different radio and telephone pathways for a president to send one-way messages to U.S. strike forces in underground missile silos, submarines and bombers.

Gerald P. Dinneen, the Pentagon's outgoing and ranking specialist on this issue, says it is this redundancy, which involves not only different paths but includes land lines, radio and satellitecommunications on a variety of frequencies, that insures that at least some will survive to carry the messages.

The Strategic Air Command, aside from having underground command posts for its force of 1,000 Minuteman land-based missiles and hundreds of bombers, also has a flying command post in the air at all times to escape attack. There are even extra rockets tucked into some Minuteman silos to be launched in an emergency, carrying radios to beep out the launch signal if other systems failed.

The Navy has shore stations that pump signals to distant submarines, and planes that constantly patrol over both oceans to send emergency signals to fire if the shore atennas are knocked out.

But hawks and doves alike have serious questions about how much of this would survive, especially the crucial first link in the chain: the ability of the president to give the order to fire or to hold fire in the wake of new developments.

The FAS warns that a single Soviet atomic weapon could pull all of Washington, including the Pentagon, the president and his 16 legally designated civilian successors out of action.

Planners express a range of concerns. One is the Soviets may believe they can either nullify or greatly reduce the U.S. ability to respond to a Soviet attack, even a relatively small one against the command centers rather than against the whole U.S. missile force.

Dinneen rejects this, pointing out that the effects of these big electromagnetic pulses aren't really very well understood, especially since there has been a ban on tests for many years.

Most important, he argues, "suppose it doesn't work." The Soviets would be giving the United States the best possible kind of warning by launching an ineffective attack upon the U.S. command network while leaving intact the vast array of U.S. missile, bomber and submarine bases.

Even a small attack also would kill thousands or even millions of Americans, and thus it would be an extraordinary gamble for Moscow to believe it would escape retaliation.

A more realistic concern, some military officers say, is that if a war starts by any means, Moscow may be able to outlast or out-negotiate the Unted States because the Soviet command and control network is generally viewed as simpler, better protected and thus better able to survive than this country's.

The Soviets, sources say, for many years had an inferior missile force at a time when the United States had a policy of massive retaliation. Thus Moscow probably expected to have to fight under attack, and gave earlier and more serious attention to both civil defense and underground headquarters.

The Soviets reportedly rely more on underground cable than above-ground microwave communications, have prefabricated new command posts coupled with equipment to dig new holes quickly, and rely on cheaper, simpler communications satellites that can be replaced easily.

Arms controller warn against the idea that limited nuclear wars can be fought. Many top officials, including Secretary of State Alexander M. Haig Jr., have agreed that once the nuclear threshold is crossed it will be very hard to control escalation.

But one top officer intimately involved in this problem says the argument should not be over whether or not there is such a thing as "limited" nuclear war. Rather, he says, planners must address the question whether any kind of nuclear attactk, maybe even one that doesn't come from Moscow, is possible in the next 20 years. If the answer is yes, he says, then the United States ought to have very sure ways of giving a measured response during a time of certain confusion.

"We've got a helluva problem," he says. "We are really in a dangerous situation the way the system is rigged now, because in a crisis our choices would be to shoot the war plan or don't," a reference to a simple choice between a major, possibly all-out, response or none.

The question of whether the command network would function in the aftermath of an atomic attack is an old one dating back to the mid-1960s when EMP effects first became worrisome and the Soviets started to build a large missile force. It received relatively little public attention in the past, however, and has become more sensitive and important recently because of the events here and in Moscow.

The Soviet missile force is now big enough and accurate enough to present more of a threat to U.S. missile silos and military command centers than in the past, though specialists differ on how real that threat is.

In this country, President Carter's issuance of so-called Presidential Directive 59 last July gave gresh impetus to the idea that the United States ought to have more flexibility in where it sends its missiles and be able to respond in kind or in different ways to any attack.

This implies being able to control, with some precision, U.S. nuclear forces after absorbing a first strike that may be less than all-out. It also implies, as officials acknowledge, that we don't have enough capability to do that now.

Other signs of this included two other recent presidential directives.

Directive 53, issued in November 1979, established a telecommunications policy that gives priority to "national security and continuity of government" in national emergencies, and calls for connection "to the maximum extent feasible" of the military and commercial systems and even including private industrial, airline and railraod communications.

Carter administration officials acknowledge, however, that relatively little has been done to follow up this directive. Part of the problem is the military. Unlike missiles and bombers, the telephones, computers and antennas needed for command and control are not very glamorous, and don't have influential constituencies in the armed services. "Nobody really wants to pay the phone bill, is what it amounts to," one top officer says.

American Telephone & Telegraph Co.'s Bell Telephone systems plays a huge role in the command and control network, with many military communications, at some point, using commercial lines. Thus the question also gets tangled up in regulations, laws and who pays the bill if AT&T or other carriers are asked to put their control centers underground for protection.

The second directive, 58, also issued last summer, is meant to improve chances for "continuity of government" by finding better ways to make sure the president and other key officials can be quickly evacuated to safe places in a crisis. Sources claim that some things have been done in this regard that now make them "more comfortable" that the president and his successors would survive.

Though President Carter is said to have taken more of a personal interest in the nuclear command situation than other presidents, and actually flew on one of his airborne command planes, several officials stressed that in an open society it is simply impossible to protect everything. They say objections by environmentalists and farmers and politicians frequently thwart projects such as the new underground antennas to communicate with submarines that the Navy has been trying to get for a decade.

William Van Cleave, who served as Ronald Reagan's senior defense adviser during the election campaign, has called the whole system of U.S. satellites that are supposed to warn of an impending attack plus the command and control system "extraordinary fragile." The lack of attention to this issue, he said, goes back a long way and is not the fault of any administration but rather a collective flaw.

Dinneen argues that the Pentagon has not been asleep on this issue and that the creation of his office four years ago and a new directorate within the Joint Chiefs of Staff 18 months ago reflects the level of internal concern and activity.

One of the president's four emergency planes, known as the E4B, has been improved with better protection against electronic pulses, with more powerful transmitters to communicate with military forces on the ground and with antennas able to link the plane directly with overhead satellites for relaying presidential instructions over even longer distances.

But officials say the program to make these improvements to the other three presidential jets, plus the fleet of SAC airborne command posts, has been dragged out. Current plans are to complete these modifications by about 1986, officials say.

The Defense Department is also developing mobile ground stations with small antennas that can be driven around the countryside to safe places and collect communications from satellites, and is also studying putting command posts in trucks.

Some military men close to the situation say they feel the situation is so big and complicated that nobody really knows how it will work in the aftermath of an attack.

These concerns run from who will be around to refuel the planes the Navy keeps aloft to communicate with missile-firing submarines to fears that the organization of the military chain of command, which hasn't changed much in 30 years, is "inadequate for political control of strategic forces in a major war."

There are also different pressures from different services. One senior officer, perhaps unfairly, says that SAC's major concern is "that it have a rope around the president's foot so they know where he is all the time and can pull the lanyard and blow up the world if need be."

During the election campaign, Carter and Reagan aides called attention to what they said was the growing vulnerability of the U.S. Minuteman missile force to Soviet missile attack.

Though some specialists question whether that is true, it may also be that, after having spent hundreds of billions of dollars over the years on missiles, submarines and bombers, that it is the system to command them that is more vulnerable than the weapons.

John Steinbrunner, a civilian specialist at the Brookings Institution, told Congress in 1979 that "The command structure of modern strategic forces is much more vulnerable to attack than are the weapons themselves. In light of this harsh fact, it seems very bizarre to spend tens of billions . . . to fix a problem that may not exist [meaning Minuteman vulnerability] while continuing to neglect a more important one [command and control] that almost certainly does exist."

Steinbrunner's reference to spending billions more because of the alleged vulnerability of the Minuteman is an allusion to Air Force plans to build the big new mobile MX long-range missile system.