DESPITE APPARENT similarities in the superpower proposals for a nuclear arms control agreement, bridging gaps between the U.S. and Soviet plans for cutting long-range, medium-range and space weapons may prove impossible.

On one major point of agreement, for example, Moscow and Washington suggest cutting by 50 percent their longest-range nuclear missiles and bombers, the so-called strategic weapons. Although the idea of halving the arsenals is broadly appealing, there are crucial disputes over which weapons would be scrapped.

Those differences partly reflect variations within the respective arsenals. The Soviets rely heavily on land-based intercontinental missiles, while the Americans place more emphasis on submarine-launched missiles and bombers.

The fundamental arguments over what is most threatening also keep changing. The president's national security affairs adviser, Robert C. McFarlane, recently said new Soviet mobile missiles dangerously challenge U.S. security.

Just a few years ago, American officials were telling Moscow and the world that such mobile missiles as the U.S. MX, originally planned as a movable weapon, would make the world more stable because they could better survive a sneak attack and were not as threatening as big, extremely accurate silo-based missiles.

The Soviets also change their minds. During negotiations in the 1970s over the SALT I and SALT II treaties, Moscow agreed to drop insistence that U.S. nuclear weapons based close to the Soviet Union -- such as short-range bombers in Turkey, Italy, West Germany and aboard U.S. aircraft carriers -- be counted as "strategic" since they could drop nuclear bombs on Soviet soil.

But Moscow recently resurrected that argument and demanded that those aircraft, along with U.S. medium-range missiles now deployed in Europe, be counted as "strategic."

In other words, arms control is an elusive, moving target.

The most significant change of heart between the superpowers is on the issue of defenses against nuclear missiles. In the 1960s, Washington argued that, if the Soviets developed a missile defense, the United States would simply build more missiles to overwhelm the defense. Now the roles are reversed, with the United States endorsing defense -- President Reagan's "Star Wars" vision -- and Moscow warning that it will respond with a surge of missile construction.

Here are the principal stumbling blocks facing U.S. and Soviet negotiators when arms talks resume in Geneva in January:

Strategic weapons -- In addition to disagreements over which kinds of long-range weapons should be cut, there are fundamental differences over how the remaining forces should be configured.

The United States wants to limit the number of warheads each superpower could have on its land-based missile force and cap the total lifting power, or throw-weight, of those missiles. Throw-weight restrictions would be difficult to verify, but Washington wants to prevent the Soviets from secretly adding extra warheads to their missiles and thus gaining a first-strike advantage.

The Soviets want only to limit warhead numbers, but they also seek tighter limits on bombers and air-launched cruise missiles.

Space -- The Soviets have made clear that there will be no deal to cut strategic missiles without agreement on restricting weapons in space.

Moscow wants limits, if not a complete ban, on Reagan's Strategic Defense Initiative, the so-called "Star Wars" research program. Reagan has adamantly resisted such restrictions, although he has offered the Soviets some access to U.S. laboratories.

Intermediate nuclear forces -- The issue of medium-range missiles, based mainly in Europe, seems to offer the most hope for agreement. Soviet leader Mikhail Gorbachev has indicated a desire for some kind of settlement, possibly because the Soviets have suffered a string of diplomatic losses since 1979 when Moscow first tried unsuccessfully to block U.S. missile deployment in Western Europe.

Moscow has proposed a freeze of both sides' intermediate nuclear weapons as of Dec. 1, followed by negotiated reductions within 18 months. The Soviet cuts would include some provisions rejected by the United States, such as allowing the number of Soviet missiles to equal the combined total of British, French and American weapons.

The United States has responded with its own proposals, and there appears to be room for negotiation, although some U.S. strategists believe that the Soviets are simply manipulating Western European hopes for a treaty.