A fundamental rethinking of US nuclear weapons and strategy has been building for months, a development that undoubtedly will be dramatized in the presidential commission report on MX missile basing.
In fact, says a government source close to the commission's inquiry, this is why members were granted a month's extension of next week's deadline for reporting to President Reagan.
Among many experts, as well as the politicians who decide such things, there is increasing concern that heavy, land-based ICBMs like the MX are (paradoxically) too threatening and too vulnerable. Their deployment, it is feared, could force a potential adversary into adopting a launch-on-warning - or perhaps first-strike - doctrine.
A decade ago, the United States was well ahead of the Soviet Union in developing multiwarhead missiles. Rather than negotiate a halt to this leap in technology, it pressed ahead. Now that the Soviet Union has caught up, the more difficult job of working back to a less threatening position presents itself.
Many think that raising the nuclear threshold (that is, lessening the likelihood of nuclear war) will require a move toward smaller, mobile, single-warhead missiles.
While this is technically possible, it would be expensive and diplomatically difficult, since it requires new arms control agreements and untried means of verification. It would, in essence, mean putting one of the genies - multiple, independently targeted warheads - back into the nuclear bottle.
Nevertheless, this approach is gaining widespread support among expert analysts and elected officials, both liberal and conservative.
William Van Cleave, director of defense and strategic studies at the University of Southern California, argues that greater superpower stability could be achieved if the US scrapped the MX and centered the land-based leg of its strategic triad on single-warhead mobile missiles of about 25,000 pounds. The MX would weigh close to 200,000 pounds and carry 10 warheads.
Dr. Van Cleave actively campaigned for Ronald Reagan and headed the administration's Pentagon transition team. In recent days, the MX commission has been seeking his advice.
''The larger the missile, the fewer and more costly the options for survivable basing,'' Van Cleave says. ''The smaller the missile, the wider the options and the lower the costs.''
The MX is too easy a target, he says. A force of smaller, truly mobile missiles, he argues, would ''greatly stabilize the situation.''
Similar arguments are made by Rep. Albert Gore (D) of Tennessee, who has been discussing with MX commission members issues much broader than finding a home for the hapless MX. These go far beyond achieving any ''margin of superiority'' or closing alleged ''windows of vulnerability.'' The basic issue is how to arrive at stability in opposing forces.
''With accurate, MIRVed ICBMs, you can have perfectly equal forces, indeed, mirror-image forces,'' Mr. Gore says. ''And yet the side going first could gain a strategic advantage, in which case the resulting relationship though equal would be quite unstable.''
Democratic senators who spend considerable time on this issue (including Dale Bumpers, Gary Hart, Sam Nunn, and John Glenn) have expressed support for a less formidable - but more stabilizing - MX replacement.
Senator Hart told the New York Council on Foreign Relations last fall, ''By equipping new missiles with only one warhead, the total number of land-based missile warheads would decline. . . . As a result, we would reinforce the importance of . . . bombers and missile-carrying submarines, which are less likely to provoke a nuclear attack.''
In a weekend newspaper column, Sen. Malcolm Wallop (R) of Wyoming called for ''a new generation of smaller, mobile, concealable, and invulnerable missiles.'' He called the MX (which he still supports, barring a better alternative) ''the brainchild of wrongheaded thinking about nuclear war.''
Richard Garwin, a physicist with many years of experience in nuclear weapons development and as a government consultant (most recently to the MX commission), says there is no way to protect a missile as big as the MX. Among other things, he advocates placing new smaller, single-warhead missiles in existing silos and aboard small submarines as a more effective deterrent to nuclear war.
In November, the Congressional Budget Office reported that even if the ''dense pack'' mode of clustering MX missiles worked, the missile would make only ''a relatively small'' contribution (5 to 13 percent) to total US strategic capabilities.
Many members of the MX commission are on record in support of the controversial and expensive missile. There have been reports that they may recommend building something less than the planned 100 missiles, deploying these in existing silos, and developing a new smaller, mobile missile.