U.S. and Soviet negotiators will sit down in Geneva next Tuesday in a renewed effort to brake the nuclear arms race, with two powerful new factors looming over the bargaining table that could push them toward agreement.

One is mostly political and bears most heavily on the Reagan administration. That is the extraordinary growth in Western Europe and the United States of popular movements demanding, in very simple terms, that the arms race halt.

The other factor is the march of U.S. military technology and weaponry that, unless checked by some agreement, has got to be causing increasing concern in Moscow.

Starting late in 1983 and continuing through the end of this decade, the United States is scheduled to deploy a series of new and highly accurate missiles and bombers that threaten to nullify the enormous investment Moscow has made in big land-based missiles. Those missiles, which have helped boost Moscow's role in global power politics, would become vulnerable as never before to American weapons.

The Strategic Arms Reduction Talks--START--are aimed at producing big reductions in the intercontinental-range missiles, warheads and bombers of both sides. If the talks succeed, they could lower the risk of nuclear war by reducing the threat that either side could militarily neutralize the other in an all-out first strike. It is that threat, however implausible, that drives the arms race.

If the talks fail and the superpowers carry through on the new weapons programs under way, it "would result in almost a doubling of strategic weapons within a decade," according to the just-retired chairman of the Joint Chiefs of Staff, Gen. David C. Jones.

Whether an unconstrained new round in the arms race would make either country's leadership feel more secure is impossible to say. But it would not give either side an advantage that would be militarily useable, says Jones.

As the accompanying chart shows, both superpowers are armed to the teeth. On balance, however, it would seem that the Soviet Union eventually could see its relative position worsen, as least in the paper calculations that are used to gauge power, if the United States pursues all the programs now under way.

The increasing accuracy of new weapons is forcing both superpowers to grapple with a problem they did not have to worry about for much of the past two decades: where to base their nuclear weapons.

In this country, this problem is illustrated most dramatically by the continued inability to find a home that makes sense and is safe from attack for the new land-based MX missile.

For the Soviet Union, however, the problem seems even greater because more than 70 percent of its nuclear striking power sits on top of what will become increasingly vulnerable land-based missiles.

The United States has more of its striking power underwater--in missile-firing submarines largely invulnerable to attack. The American submarines and the missiles they carry, as well as those in prospect for the future, are viewed as superior to their counterparts in the Soviet undersea armada.

Therein lies a major, future dilemma for Moscow. If its land-based missiles become vulnerable to attack from new American missiles, should the Kremlin invest billions more rubles to shift more forces to sea, an operating arena where the United States already has a technological advantage?

It is questions such as these that could figure prominently in the new talks and that give at least some U.S. officials the feeling that conditions are present that eventually could lead to some agreement.

In Washington's view, there are other conditions that also may make Moscow interested.

Although experts debate the extent and impact of Moscow's current economic woes on Soviet policy, the White House seems convinced that the Soviets have serious financial problems, and President Reagan, sources say, is intrigued by this.

Soviet President Leonid I. Brezhnev is also nearing the end of his reign, and it is widely held here that the Soviet chief does want some sort of new arms agreement, and a return to an earlier form of detente in which access to American technology, trade and credit were easier.

But there are also problems emanating from Washington. The Reagan administration's sometimes strident rhetoric about nuclear matters has created much of the political protest that is pressing in on its policies.

The president, on March 31, also claimed that "on balance, the Soviet Union does have a definite margin of superiority" in strategic striking power. While many former officials and specialists disagree, the president's view is reflected in a START proposal that would require Moscow to make much greater cuts in its forces than Washington must make. That, too, is a new factor because the two powers had approached all previous arms negotiations on the basis that a rough parity existed.

Reagan's proposal calls for numerical equality. Both sides would reduce the total number of individual missile warheads by roughly one-third to a level of 5,000 each. No more than half of these could be on land-based missiles, and each side would be allowed 850 land- or submarine-based missiles.

The idea is to force upon Moscow a sizeable cutback in warheads on their land-based missiles, which are the most accurate and, therefore, the most threatening.

Reagan wants to concentrate first on reducing these potential first-strike weapons because they are also the most vulnerable to attack and thus have a "use them or lose them" quality.

Brezhnev has rejected Reagan's proposals as "lopsided" and "unrealistic," aimed at protecting U.S. advantages in bombers, submarine missiles and cruise missiles, and meant to tip the current balance of power. He has, nevertheless, agreed to the talks and the idea of eventual reductions, but first proposes an immediate freeze that would ban new weapons.

Even U.S. officials who are optimistic about some agreement in the long run say the chances for quick success at Geneva, perhaps over the next year, are "almost nonexistent."

They expect Moscow to continue trying to manipulate public opinion, especially in Western Europe, to force a U.S. backdown on key points and to wait and see how the freeze movement grows in this country. They also expect the Soviets not to budge from their initial position while the future of the MX remains in doubt.

On the other hand, top officials here say Reagan wants progress on START before he agrees to a summit with Brezhnev, which the Soviet leader has proposed for this fall.

Paradoxically, some officials believe it could be the scheduled December, 1983, deployment of new U.S. Pershing II and cruise missiles in Europe--missiles that are covered in a separate negotiation and not in START--that breaks open the arms control logjam.

These weapons, meant to balance some 300 Soviet SS20 missiles in place, are being discussed in the Intermediate-range Nuclear Force (INF) talks dealing with European-based weapons that began in Geneva last November.

The U.S. missiles fit the pattern of increased threats to Soviet forces and command posts. The Pershing, in particular, can reach Soviet territory and supposedly hit targets with high accuracy and little warning.

It is widely assumed among many U.S. and Soviet specialists that at some point the START and INF talks will be merged.

There has been no real progress on INF, officials say. The Soviets claim a balance in European-based power already exists. They argue that the British and French have roughly 300 missiles, about the same as the number of SS20s, so there is no reason for new American weapons.

The U.S. strategy at START, officials say, will be to concentrate on getting the Soviets to accept the idea that land-based missiles are the most politically destabilizing weapons and must be negotiated first.

The Soviets are certain to immediately point out that the United States has a larger bomber force and is building 3,000 to 4,000 new long-range cruise missiles to put on those bombers, plus several hundred more cruise missiles for ships and submarines.

The U.S. response is that these weapons can take hours to reach their targets and thus are not the crucial problem. The administration has said it will "deal" with these weapons but does not want to negotiate about them in the initial phase.

Furthermore, when bombers do come up for discussion, the United States will point out that the Soviets have massive air defense systems that must be included in the calculations. The United States is also seeking to count the Backfire bomber as a long-range weapon even though Moscow says it is only for regional use.

When cruise missiles come up, Washington will point out that Moscow also has a good number of them, even though they are mostly of a few hundred miles in range and primarily useful for attacks on shipping.

Aside from wanting to stop the Pershing deployment, the big prizes for the Soviets would be curbs on the MX missile and the new Trident II submarine-based missile now in development.

Although the fate of the MX has grabbed the headlines in recent years, the Trident II is perhaps the most important new weapons development of this decade, one that could alter future forces and strategies.

This is so because, if it works as advertised, the missile, for the first time, would give submarine-based weapons sufficient accuracy to knock out Soviet missiles in protected underground shelters. Strategically, this means both an invulnerable and effective force.

A key element of American START strategy, however, is that nothing in the U.S. proposal would prevent fielding MX, Trident II or the new B1 and Stealth bombers. Under this idea, if both sides agree to reduce the level of forces, they are free to deploy whichever weapons they want to make up those smaller forces.

Thus, unless the Soviets offer a deal that is too good to turn down, U.S. officials say MX is not up for grabs. So, while the overall numerical threat from each other's forces may be reduced, the arms race--in terms of the U.S. taxpayer--will probably not come to an end.

Some specialists have suggested the U.S. propose a deal in which MX is dropped in return for the Soviets removing their 308 big SS18 missiles. But administration officials say such ideas were discarded before they ever got to Reagan.

Officials claim even if there were such a deal, it would still leave Moscow with a unilateral advantage in big missiles because even the next biggest Soviet missile, the SS19, is significantly larger than the existing U.S. Minuteman III.

It is three years since former president Carter and Brezhnev signed the second strategic arms limitation agreement, SALT II, which was supposed to put a ceiling on the strategic forces of both sides at roughly current levels. But that treaty was never ratified here for a variety of reasons, including the Soviet invasion of Afghanistan.

Aside from the Reagan emphasis on deep reductions rather than limitations, there are other major differences between SALT and America's opening START position.

They involve removal of bombers and the weapons they carry from the initial calculations, and a focus on the actual number of missiles and warheads in the field rather than the number of "launchers" in which missiles are housed.

Addressing the United Nations session on disarmament last week, Reagan mentioned the need for "effective verification" of these complex agreements eight times in his speech.

Yet officials say the United States has only just begun to do the detailed work necessary to figure out how to verify possible agreements. Like a lot of things about the negotiations that open next week, American officials say figuring out precisely what will be necessary depends on what the Soviets say and do.