A combination of congressional pressure, arms control politicking and Pentagon concern about improving Soviet rocketry is pushing the Carter administration toward acquisition of a new rocket system that could change the nature of the Soviet-American strategic relationship, according to numerous official sources.

The most ardent proponents of the new missile system, called MX, argue that it is urgently needed to compensate for advances in Soviet capabilities.

Move moderate defenders of the MX contend that it is a necessary step to demonstrate to the Soviets that the United States will continue to improve its weapons while negotiating new strategic arms limitation treaties (SALT).

Critics of the new missile system argue that it could lead to a dangerous and abrupt escalation of the arms race, conceivably undermining the foundation of all current and foreseeable arms control agreements with the Soviet Union.

Despite uncertainty about its potential impact, though, planning for the MX is proceeding apace, with the relevant congressional committees and the administration acting as though full-scale development is to begin.

Last Saturday the Air Force made a formal presentation to the Pentagon's Defense Systems Acquisition Review Council (DSARC) outlining its desires for the MX. Defense Secretary Harold Brown has said he will soon decide what he wants in the way of an MX system.

Last week in closed session the Senate Armed Services Committee approved a supplemental authorization including $265 million for MX development and decided to inform the administration that the committee hopes to make definitive decisions on the MX in May.

The MX has also become a factor in the debate over the new SALT II pact with the Soviet Union, with many senators prepared to demand development of the missile as a price for supporting SALT. Other senators threaten to vote against SALT if the MX is developed.

The MX debate involves both the missile - which would be bigger and carry more thermonuclear warheads (10) than any previous land-based American intercontinental ballistic missile - and the way in which it would be deployed. The method of deployment is the subject of the hottest dispute in the government, since administration officals tend to see development of an MX system as inevitable.

The Air Force has pushed for deployment based on thousands of holes in the ground-missile silos. The missile itself would be trucked around a large area containing holes, and inserted in silos at random but at a ratio of more than 20 empty, or decoy, holes for every one containing a missile Current Air Force plans call for 200 MX missiles scattered in 4500 holes. Deception would be aided by transporting the missiles in huge canisters that might - or might not - contain an actual rocket.

An alternative system would involve the construction of a new airplane that could marry Mx missiles. These planes would be designed to land on runways as short as 3,000 feet. They could take off in a crisis to avoid being hit by Soviet rockets while on the ground, and could land at any of 4,500 designated, undamaged landing fields, then could quickly fire their rockets at the Soviet Union.

In its report on the MX delivered Saturday to top Pentagon officials, the Air Force said the airborne deployment system would cost $29 billion to install and $900 million a year to maintain. The shell-game scheme, the Air Force said, would be nearly 50 percent cheaper.

The idea of the multihole or "shell game" deployment is to force the Soviets to target their weapons on all 4,500 silos, even though only 200 would contain American missiles.This many silos could strain Soviet capabilities beyond their limits, making it impossible for Moscow to wipe out America's land-based missiles with a single surprise attack.

A problem with this scheme, its critics contend, is that such a deceptive basing system would make it difficult if not impossible for the Soviets to know how many rockets the United States had deployed, since Soviet satellites could not differentiate between full and empty silos.

Proponents of the shell game reply that methods could be found to periodically open the silos so the Soviets could look and count the deployed rockets.

Critics respond that if the Soviets copy the shell game system, then the United States would face an insoluble verification problem, and would also face the danger that in a crisis the Soviets could rapidly bring rockets out of storage and fill up all the empty silos.

This is the nighmarish scenario embraced by MX's harshest critics. If both superpowers end up deploying deceptive, mobile missile systems, they argue, there will be little hope of continuing a SALT process that depends on each country being able to verify with confidence the other's strategic programs. Moreover, the danger of one side miscalculating and firing first in a crisis could increase.

Proponents of MX question whether the Soviets would try to copy the shell game, though many acknowledge that the Soviets seem inclined to proceed with development of some sort of mobile missile system. Many argue that the United States must keep up with Soviets even if they choose a more dangerous course for the strategic competition.

(The SALT pact that is nearly complete includes a protocol banning mobile missile systems, but it would be in effort for just three years.)

The verifiability problems has persuaded some administration officials some administration officials that a new MX should be deployed only on airplanes. But proponents of the shell game note that it is cheaper and less vulnerable to a sneak attack planes on the ground.

As a possible response to this argument, the Air Force experts at Saturdayhs secret Pentagon meeting discussed a new option, mixing airborne MXs with permanently fixed landbased missiles in well-protected silos.

This combination could be built for slightly less than the all-airborne system, the experts said, and could be less vulnerable to sneak attack too, because the protected, land-bsed missiles could survive the most dangerous sort of sneak attack, one launched by relatively inaccurate Soviet submarines off the U.S. coastlines.

Another argument over the MX concerns its potential as a "first strike" weapon that the Soviet might fear could be used against them in a devastating sneak attack to which they would have no adequate response. As envisioned now, each MX would carr 10 thermonuclear warheads that could each be directed at individual targets with unparalleled accuracy-good enough to drop war-heads right into Soviet missile silos.

Proponents of the MX say the Soviets are developing what amounts to a first strike capability, so they cannot expect the United States to do otherwise. Other say the number of MX can be held to 200, which would give the United States 2,000 MX warheads-not enough to threaten the Soviets with a disarming sneak attack.

Until the last six months or so, the Carter administration proceeded on the assumption that it could complete SALT II and get final Senate action on it before making a decision on MX development and deployment.

But SALT negotiations have dragged on month after month, and various pressures inside the Pentagon and on Capitol Hill have continued to push MX forward. Even if the most optimistic administration timetable is now met, and the Senate begins deliberating on SALT II thissummer, it appears inevitable that the SALT debate and the congressional discussion of MX would go virtually in tandem.

Senior administration officals-including some who say privately that they think MX is unnecessary and dangerous from a strictly military point of view-argue that Congress and the country will demand missile modernization, including MX, as a guarantee that the United States will remain secure with SALT.

Defense Secretary Brown, government sources say, privately seems to be leaning toward an air-mobile MX to avoid the potential pitfalls of the shell game system.

President Carter, it is reliably said, sees the seriousness of his MX dilemma but hasn't yet signaled how he may cope with it.

"If Carter got an option paper that said, 'Do you want a first-strike missile system?' or 'Do you want a system that will undermine future verification of SALT agreements?" I'm sure he would say no to both," one senior official said. "But Carter may not get the question in those terms. He may not be able to avoid it," the same source-no friend of MX-added.

But a number of senators, including George Mcgovern (D-S.D.), Edward M. Kennedy (D-Mass.) and Mark Hatfield (R-Ore.), feel strongly that MX should not be built, and particularly not for a shell-game deployment. They contend that this would dangerously destabilize the arms race and jeopardize the entire SALT process.