OF ALL THE political axioms of Washington, one we often forget is to "watch what they do, not what they say." This is particularly worth remembering today in the area of national defense.

President Reagan, Defense Secretary Weinberger and others have, of course, been saying a great deal about defense, and the president no doubt will speak again in his State of the Union address about how determined he is to strengthen our military capability.

Those who watch instead of listen, however, will make an intriguing discovery: The Reagan administration in the past year actually presided over a significant reduction in strategic nuclear weapons on alert as well as in the development of new weapons for the future.

Item: Beginning Oct. 1, the United States had only 31 ballistic missile submarines in operation, eight less than the year before and 10 fewer than allowed under the SALT I arms limitation agreement with the Soviet Union.

Item: There was but one American B52 armed with air-launched Cruise missiles which was ready to fly. The first squadron, originally planned to go on alert Sept. 1, 1981, will not be ready until December of this year.

Item: One-third of America's nuclear explosive capability -- sitting on aging, Titan II missiles -- has been earmarked for retirement beginning next year. That's four years ahead of the Carter administration schedule and well before 1986, when the first of their replacement missiles, the MX, are to be deployed.

Item: No significant acceleration is planned by the administration for the D5 Trident II missile, the next generation of sub-launched missile, which looks now to be the weapon on which Weinberger is basing his future strategic program.

Item: This lack of acceleration on the D5 comes despite what is perhaps Reagan's largest arms reduction action: his decision to halve former President Carter's plan for 200 MX missiles to 100, and then to put just 40 in silos by 1988.

Item: The only new weapon in the Reagan plan, the B1 bomber, will now arrive in 1987, but it will make no significant difference even then in the basic U.S.-Soviet strategic balance of nuclear forces. The still undefined increase in Cruise missiles, to be put on bombers and aboard ships, also will have little effect on the basic nuclear balance.

In short, if a Democrat were in the White House today, conservative Republicans almost certainly would be accusing him of unilateral disarmament.

Much of the Reagan defense program, of course, is being driven by budget dilemmas. Despite big increases in Pentagon spending levels, the added dollars are unable to keep pace with more rapidly escalating costs.

The situation wasn't helped, moreover, by the Carter administration's underestimates of bills for major defense programs like the MX. Carter projected costs of $35 billion for 200 missiles and 4,600 shelters, a figure the General Accounting Office termed far too optimistic. The reduced Reagan plan will cost $19.4 billion to buy 100 missiles and base only the 40.

As a result, some defense experts inside and outside the administration believe, Secretary Weinberger is undertaking a surprising gamble for now, leaving himself the option of changing course next year. He appears to be hoping, they calculate, that at least the Soviets will overlook Washington's words and concentrate instead on what he is doing -- or, more to the point, not doing. Then, these officials suggest, the hope is that the Soviets, too, will cut back on their expensive and threatening land-based missile program.

One administration official, for example, noted that although the Soviets have at least one large, land-based ICBM model ready for testing and another new, solid-fuel mobile ICBM, neither has been fired off.

Instead, for the past several months, this source said, the Soviets have been concentrating their tests on a large new missile for the Typhoon submarine, which is much like our Trident. Sub-launched missiles, this source emphasized, are unlikely to be first- strike weapons.

Another top Pentagon official remarked recently in private that he hoped when the Russians do test a new land-based ICBM, it will be the mobile one, which he said would also be less threatening to the United States.

Beneath their concerns is the fact that the Reagan cutback in the MX program did more than reduce the number of missiles planned for deployment. It undermined the strategy put forward to justify the new ICBMs in the first place.

Carter's 200 MXs, each with 10 warheads, were to be the basis for a new nuclear doctrine -- the so-called PD-59 announced in August 1980. The missiles were to provide a force which not only could survive a Soviet first strike -- because they were to be rotated among 4,600 shelters -- but which also could retaliate in numbers capable of destroying the silo-based missiles the Russians had left.

This was all mystical to most people and nonsense to others, but to the true believers in nuclear strategy, PD-59 was a doctrine to be devoured and argued over endlessly.

But if Secretary Weinberger now puts only 40 MXs in silos, they will be just as vulnerable to a Soviet strike as the missiles we have today. And you can forget the talk about hardening the silos: It can't be done, and the administration may not even try.

Where does that leave PD-59 and all the other war-fighting strategies that the Reaganites have talked about for years?

"There has been considerable uncertainty about the substance of the Reagan administration's strategic policy," was the generous way that question was answered recently in an article by Donald M. Kerr, director of the Los Alamos National Laboratory, and Robert H. Kupperman, executive director of Georgetown University's Center for Strategic and International Studies.

These two authorities on nuclear strategy say that the presidential campaign and the Reagan transition gave the appearance that "a Reagan administration would react strongly, if not overreact, to the perceived Soviet threat and the so-called 'window of vulnerability.'" Instead, they write, the president's programs to date "have gratified neither hawk nor dove."

Unavoidably, this Reagan defense dichotomy has produced a similar mismatch between rhetoric and reality on the opposite side of the defense coin -- arms control.

A basic criticism of the SALT II arms limitation treaty, voiced by candidate Reagan and many of those now high in his administration, was that the Carter treaty left the Soviets with a major advantage in the amount of nuclear power -- "megatonnage" in defense parlance -- they could deliver on the United States. Because the Soviets had bigger missiles with bigger and thus more powerful warheads, this argument went, the United States under SALT II would remain at a permanent disadvantage. The treaty, after all, limited the number of launchers and warheads each side could have without also limiting their size.

To correct this, Reagan often said during his campaign that he would negotiate with the Soviets next time only from a position of strength -- meaning adding megatonnage to U.S. nuclear forces before new talks began. But his unilateral decision to halve the number of MX missiles he plans to build has instead cut the originally anticipated increase in U.S. megatonnage. At the same time, moreover, Reagan's decision to retire the Titan II missiles, which carry this country's biggest warheads, will sharply reduce existing U.S. megatonnage.

How can Reagan conceivable negotiate then from a position of strength? How can his negotiators reach some agreement with the Soviets on megatonnage if the president's actions will increase Moscow's lead in this area of land-based ICBMs to nearly five times that of the United States?

The answer seems to be that the administration will come up with a radical new form of measuring comparative nuclear strength, that it will not use the single standard which it termed the most important yardstick just a year ago. Surprising as it may seem, two conservative Reagan officials in the defense field within the last month have said privately that such notions -- again altering rhetoric rather than reality -- are currently being discussed.

One approach, sources said, is to use the concept of "survivable megatonnage," an idea built on calculations put together during the SALT II debate by Paul Nitze, who is now Reagan's negotiator with the Soviets on intermediate-range missiles. This concept measures the megatonnage on each side that could survive a first strike.

The United States does better using this approach, because so many more of our warheads are on survivable submarine missiles, while three-quarters of Soviet warheads are on land-based missiles that would be vulnerable to an American attack. But how this could be translated to an arms control approach remains to be seen.

What is clear is that all of this is frustrating to former Carter defense officials who were ruthlessly attacked as being weak and whose SALT II treaty was unable to pass the Senate. Former Defense Secretary Harold Brown has bitterly criticized the Reagan defense cutbacks, but his criticisms have had none of the impact that Reagan attacks on Carter policies generated just over a year ago.

The plain fact is that conservative Republicans have much greater leeway for action -- or inaction -- in the nuclear weapons field than do Democrats.

Remember that it was Henry Kissinger and Richard Nixon who in 1972 traded away a U.S. numerical advantage in sub-launched ballistic missiles to get a SALT I agreement. It was also Kissinger and Nixon, in the same treaty, who accepted a definition for a new Soviet ICBM that was broad enough to permit Moscow to almost double the size of its new weapon and thereby move far ahead of the United States.

Back then, the Nixon administration had few defense critics on their right, and the Democrats were fully ready to support whatever arms control agreement they could get. The same situation exists today for Reagan and Weinberger.

What is different today, however, is the verbal gap that the president and defense secretary have created. They are doing one thing, cutting strategic nuclear forces, and saying the opposite. Are they fooling themselves? Or is this a sophisticated plan, based on the hope that the Russians will cut back also?

Secretary Weinberger has positioned himself so that he can still change gear in July 1983, when he is to decide what to do with the 60 additional MX missiles scheduled to be built. By then, he will have had a look at the new Soviet missile tests and, hopefully, we will have had two years of new strategic arms reduction talks in Geneva.

If the Russians are tough on arms control and have tested a big new missile, the defense secretary could push the U.S. program into high gear, perhaps even reviving the mobile- based MX of Carter and throwing in an anti- ballistic missile system.

But if the Soviets remain restrained in their arms building and flexible on arms control, who knows what could happen under an administration that has pledged to be tough and acted in the opposite manner?