Washington — Faced with a dramatic decline in public support for increased defense spending, the Reagan administration is mounting a public-relations offensive to save the Pentagon's largest-ever peacetime buildup.
Experts are being brought off the bench from prior administrations and recalled from diplomatic posts to lobby Congress and the public. In his radio talk over the weekend and in a speech to the American Legion Tuesday, President Reagan warned that the buildup is necessary to counter Soviet military might and aggressiveness. The US has ''an overriding moral obligation to do so,'' he said.
Last year's Defense Department report on ''Soviet Military Power'' shortly will be updated with new information showing that Warsaw Pact military investment (weapons procurement, research and development, and military construction) clearly outpaces that of the West. What US officials find particularly troubling is that Soviet forces are ''increasingly structured for offensive use.'' This concern lay behind a special classified briefing for selected reporters at the Pentagon in December.
Such themes are not new to Mr. Reagan. But he's finding it difficult to hold the support of Americans and allies. This is why Vice-President George Bush was dispatched to Europe recently. Here at home, the proportion of Americans who favor increased defense spending has dropped from 71 percent in 1980 to 17 percent now, according to pollster Louis Harris. With few exceptions, Republican and Democratic lawmakers are calling for cuts in the Reagan defense buildup.
Reagan's more hawkish friends complain that the President has failed to publicize sufficiently the necessity of his buildup. Reagan's response comes just as Congress takes up the 1984 Pentagon budget, which shows the steepest increase (10.3 percent) of all planned military budgets through 1988.
According to published documents and confidential information, the Reagan administration sees the following trends in Soviet military power:
* In 1966, the Soviet Union and the United States were spending nearly the same (about $70 billion) on weapons, military installations, and related scientific research. By 1981, while the US was still spending the same amount in these areas (not counting inflation), the Soviet Union was spending more than $ 120 billion.
When Warsaw Pact countries and NATO/Japan are added, the figures are less disparate. But the US and its allies still are behind by about $15 billion and have been since 1971. The Warsaw Pact advantage widens with the greater standardization and economies of scale it enjoys.
* In all categories of weapons production, except major surface ships, the Warsaw Pact leads NATO. At the same time, the Soviet Navy has advanced from a primarily coastal defense force to one with true ''blue water'' capability, extending most recently into the Pacific. It has launched its third Kiev-class aircraft carrier, and its giant 25,000-ton Typhoon ballistic missile submarine (considerably larger than any US sub) is approaching operational service.
* The Soviet Union has gained increasing access to bases outside its borders, including Cuba, Vietnam, South Yemen, and Angola.
''The ultimate Soviet goal in Europe,'' Reagan told the American Legion's annual Washington conference this week, ''is to force the nations to accommodate themselves to Soviet interests on Soviet terms.''
Critics argue that NATO is a much stronger alliance than the Warsaw Pact, and that the West continues to have a technological edge (although it is acknowledged that this is slipping). But the International Institute for Strategic Studies (IISS) in London tends to support the administration view.
''The numerical balance over the last 20 years has slowly but steadily moved in favor of the East,'' the IISS states. ''At the same time the West has largely lost the technological edge which allowed NATO to believe that quality could substitute for numbers. One cannot necessarily conclude from this that NATO would suffer defeat in war, but one can conclude that there has been sufficient danger in the trend to require remedies.''
Production of selected weapons n1 -- 1974-82 Soviet Non- Pact to Soviet Non- to Soviet US Warsaw US NATO Category Union US Ratio Pact NATO Ratio Tanks 17,350 6,400 2.7:1 3,450 2,600 2.3:1 Other armored vehicles 2 36,650 4,800 7.6:1 9,100 10,300 3.0:1 Artillery and rocket launchers 13,350 350 38.1:1 1,300 700 14.0:1 Tactical combat aircraft 3 6,100 3,050 2.0:1 800 2,650 1.2:1 Intercontinental ballistic missiles 2,035 346 5.9:1 - - - Major surface warships 85 72 1.2:1 10 79 0.6:1 Attack submarines 61 27 2.3:1 - 33 1.0:1 Ballistic missile submarines 33 2 16.5:1 - 3 6.6:1 Theater nuclear missiles 4 5,850 3,550 1.6:1 - 1,450 1.2:1
n1 Totals represent that portion of nation's production earmarked for its own military services plus imports and excludes production for export.
n2 Includes light tanks; armored personnel carrier; infantry fighting vehicle; reconnaissance, fire support, and air defense vehicles.
n3 Includes fighter, attack, reconnaisannce, electronic, electronic warfare, and all combat-capable tactical training aircraft.
n4 Includes ground- and sea-launched missiles. Also intermediate- and meduim-range missiles. Source: Pentagon's Annual Report to Congress -- FY1984