Washington — In the two most important areas of its nuclear weapons and arms control programs, the Reagan administration this week finds itself walking a fine political line.
* It will be several weeks before Congress says yea or nay on the MX, but already it looks like an uphill roll for the big intercontinental ballistic missile. Key lawmakers are skeptical of the administration's promise to develop a new smaller and less threatening missile and to seek new initiatives in arms control as the price for obtaining the MX. This has put the White House on the defensive.
* In responding to a new offer by Soviet leader Yuri Andropov on intermediate-range nuclear weapons in Europe, American officials must express their criticisms and doubts so as not to appear too intransigent to the increasingly restive and popular peace movement in Europe. At the same time, the administration cannot stray too far from NATO's own position on arms control and new weapons deployment on the continent, particularly since alliance governments in recent months have pulled together on the issue.
Mr. Andropov's offer to count warheads as well as launchers in Europe was welcomed as ''progress'' by US officials. But the new position (to be discussed when arms negotiators return to Geneva later this month) also leaves several important problems unresolved.
The Soviets still want to count French and British strategic weapons, which NATO insists must remain independent. The Soviets say nothing new about the many SS-20 missiles deployed in Asia that could be quickly moved westward in times of tension, nor do they say whether missiles now aimed at Western Europe would be destroyed or simply moved east. They apparently want to count NATO's ''dual-capable'' (conventional and nuclear) aircraft, which the alliance rejects. And they do not specifically account for the fact that SS-20s have a reload capability and extra missiles deployed.
''The idea of agreeing to count warheads is a good thing,'' Defense Secretary Caspar W. Weinberger said at a news conference Wednesday. ''I think that's encouraging. But there are certain immediate problems with it that are apparent on the surface.''
The Pentagon chief this week is urging Congress to restore funds for the Pershing II missile, which will be NATO's answer to the Soviet SS-20s if an arms control agreement is not reached before the end of the year. Lawmakers last year held up funding for the missile pending successful test results.
Mr. Weinberger noted that the Pershing has had seven successful test flights since then, and said congressional cooperation was ''critical'' for allied unity and arms talks with the Soviets.
The Andropov offer to count warheads instead of launchers in Europe parallels the recent MX commission recommendation to move in this direction on strategic arms control, and administration assurances this week that the current START proposal will be reexamined with this in mind.
On Capitol Hill, MX commission members and senior Air Force officers are trying to convince questioners that deploying 100 missiles with a total of 1,000 warheads will not move the US closer to striking first or adopting a destabilizing ''launch-on-warning'' posture. Instead, they say, it would deter the Soviet Union from launching its many highly accurate, first-strike-capable weapons.
This perception of the MX's warfighting ability is likely to be the key to its acceptance or rejection by the Congress. Former CIA director Stansfield Turner and former presidential security adviser McGeorge Bundy this week testified against the MX on the grounds that it would reduce nuclear stability and make war more likely.
Spokesmen say the administration is now studying how its strategic arms reduction proposal can be modified to reflect the new emphasis on counting warheads rather than launchers. And the Air Force this week announced that it is opening a new office at Norton Air Force Base in California for research and development of the new single-warhead missile.
But the MX is still first priority for the Air Force, the administration, and the presidential commission that was chaired by retired Air Force Gen. Brent Scowcroft. In order to deter a Soviet nuclear attack, it is argued, the US must have the same ''prompt hard-target kill capability'' as the Soviets. This means very accurate ICBMs that can destroy an opponent's land-based missiles and command facilities.
''A silo-based Peacekeeper (MX missile) will resolve this fundamental and destabilizing imbalance by placing at risk the Soviets' newest and most lethal strategic assets,'' the Strategic Air Command's commander-in-chief, Gen. B. L. Davis, testified recently. ''In effect, we need the Peacekeeper to blunt the Soviets' warfighting advantage so that they could never believe they would prevail in a nuclear conflict.''
As explained in the Scowcroft commission report, adding the MX to the US nuclear triad of land-, sea-, and air-based weapons would present any attacker with insurmountable timing problems and uncertainty of success. The Congressional Budget Office this week reported that the Soviet Union might be deterred from attack by just such uncertainty. But it also noted that unless the missiles were launched before being attacked, fewer than 10 percent of them would survive.
Congress now must ask itself whether uncertainty about the MX's military value is worth the uncertainty it could present for the Soviet Union.