This week sets the scene for a fundamental examination of the basis for President Reagan's push to rearm America. The nuclear freeze movement sweeps back into town with public demonstrations. Forces on Capitol Hill normally friendly to the Pentagon continue the legislative move to slow the rate of the proposed military buildup. The Pentagon releases a new edition of its dark and troubling report, ''Soviet Military Power.''
At this key juncture, United States intelligence officials now concede that the pace of Soviet military force growth in recent years has been slower than earlier thought. This gets to the heart of Mr. Reagan's rationale for enduring huge deficits while pouring record sums into new weapons. He's argued that defense spending shouldn't be measured against economic impact or social goals but against the size and nature of the Soviet threat.
Until recently, US intelligence analysts assumed that Soviet military spending has grown 3 to 5 percent a year (not counting inflation), which would have been faster than the US. Now, they find that the figure for 1976-81 actually was closer to 2 percent, more in line with the rate of increase in defense spending in the US.
''For a while, we thought we were looking at a short-term cyclical phenomenon ,'' says a senior specialist on Soviet affairs. ''This past fall, we began to realize it was longer term.''
Central Intelligence Agency (CIA) and Defense Intelligence Agency (DIA) officials say they are now trying to figure out why the current flattening in rate of military growth occurred and how long it is likely to last. There were similar occurrences in the late 1950s and late 1960s.
Intelligence experts critical of the way government analysts figure Soviet defense spending say there are problems with the government's methods that can distort estimates: converting rubles to dollars, for example, and judging personnel costs as if Red Army troops were paid as well as US soldiers.
''I'm not surprised that they feel they've overestimated Soviet spending,'' says Franklyn Holzman of Tufts University, who has written extensively on the subject and has worked with government intelligence analysts. ''They always make the worst-case assumptions.''
While acknowledging that they must reprogram their computers to show a slower rate of Soviet military growth, US intelligence officials warn that this country remains well behind the Soviet Union in important areas of military capability. These include construction of tanks, aircraft, submarines, and nuclear missiles.
At the same time, however, the slower rate at which these weapons are built may be tied to Soviet economic difficulties, officials say, as well as problems arising with new high-technology systems. As in the US, moving toward more complex weapons apparently means having to build fewer of them.
What CIA and DIA officials stress (and what will be emphasized in the Soviet Military Power report this week) is that there has been no slowdown in key aspects of Soviet military investment. This includes research, development, and construction of weapons production facilities. While the two agencies are not always in agreement, both say the Soviet Union is spending about 70 percent more than the US on military R&D.
''That's a frightening gap,'' says one specialist. ''The Soviet defense establishment is still very intact, still very much healthy.
''Their production facilities have been expanding rapidly, especially in the past few years. It may indicate that there will be a new burst of production in the next few years. That's what we're trying to figure out.''
The key is thus: How reliable are these projections if it is now acknowledged that rates of overall Soviet military growth have had to be adjusted downward?
In a report to Congress last week, the Pentagon said that while the Soviet Union is closing the gap in some instances, the US still enjoys a notable margin of superiority in basic technologies ''having the potential for significantly changing the military balance in the next 10 to 20 years.''
All of this is likely to raise other important points: How good are Soviet weapons and how formidable are Red Army troops?
US and Israeli weapons dramatically outperformed Soviet-made weapons wielded by Syrian forces in Lebanon last summer.
It was reported recently that US monitoring of Soviet missile tests show these missiles to be considerably less accurate than administration officials have warned. Weapons expert Kosta Tsipis of MIT soon will issue findings that reach the same conclusion about the alleged Soviet ''first strike'' capability. Critics site such reports in raising questions about the necessity for the MX missile, designed to be able to hit Soviet strategic forces and command structures.
For some time, Rep. Les Aspin (D) of Wisconsin, a former Pentagon analyst, has been using US intelligence data to illustrate growing problems with Soviet troops. Similar points are made by an officer writing in the current issue of Air Force Magazine.
''The problems affecting the capabilities of the Soviet conscript, and hence the Soviet armed forces, are numerous,'' writes Air Force Capt. Alan J. Bergstrom. Among these: high turnover (25 percent every six months), low morale, projected shortages of draft-age youth, ethnic imbalances presenting language and culture difficulties, and ''chronic'' desertion, insubordination, and excessive use of alcohol.
Like a field full of tank traps, such points - together with the new US intelligence information on Soviet defense increases - could make it more difficult for the Reagan administration to prevail as the defense debate continues.