President Reagan yesterday skillfully wove together favorite themes of America's allies overseas and American conservatives and liberals at home in an arms control speech intended to keep the Soviets off balance and the NATO alliance plan on course for countering Soviet missile power in Europe.

For allies in western Europe whose governments still face popular opposition to deployment of new U.S. missiles there beginning in December, the president's new proposal for a negotiated settlement with the Soviets sustained the idea that Washington, not Moscow, is striving for an accord.

For conservatives who have criticized Reagan for continuing arms talks after the Soviet downing of a South Korean passenger jet, the president artfully linked the airliner incident to a strong assertion that the Soviets cannot be trusted to comply with any agreement unless there is an airtight verification process, something conservatives have demanded.

For congressional moderates and liberals who have urged that U.S. proposals be as negotiable as possible in hopes of capping the arms race, the president's latest offering represents several potentially important modifications of earlier U.S. positions.

Whether there is anything in these new proposals for Moscow, however, is the key question.

The Soviets have snubbed the latest Reagan plan as nothing new.

Indeed, U.S. and allied officials are convinced that there is virtually no chance that Moscow will agree to a pact limiting medium-range nuclear missiles on both sides until after the West proves its ability to weather demonstrations likely this fall in western Europe and begin deployment.

The Soviets are also unlikely to sign anything prior to a unilateral western deployment that would countenance even a single U.S. missile being added with their approval, officials have said.

Nevertheless, administration specialists believe the latest Reagan proposal breaks new ground in several areas and could provide the basis for more fruitful negotiations some time next year after the crucible of beginning deployment is passed.

One official said yesterday that "whether this is going to help or not, no one can say. But it certainly doesn't hinder progress."

In December, the United States and its NATO allies are committed to begin fielding the first of 572 new medium-range U.S. Pershing II and cruise missiles in West Germany, Italy and Great Britain unless the Soviets agree to a negotiated settlement at the Intermediate Nuclear Forces (INF) talks in Geneva.

Under such a settlement, Moscow would have to scrap many of about 600 medium-range missiles it has deployed in the European and Asian portions of the Soviet Union. Of those weapons, the 351 modern SS20 missiles, each of which is mobile, pose the most serious military and political intimidation threat to allied targets in Europe and Asia.

Of the 351 SS20s, a reported 243 are based within range of Europe and 108 within range of Asia. Because each Soviet missile carries three warheads and each U.S. missile carries one, the U.S. proposal seeks to achieve equal numbers of warheads on both sides. This would mean allowing Moscow fewer missiles.

At the start of the INF talks in November, 1981, Reagan proposed a so-called "zero-zero" plan in which the United States would forgo all 572 new missiles if the Soviets dismantled all of their forces.

Last March, that plan was modified, largely in response to allied pressure. The interim offering allowed both sides equal levels of missile warheads, somewhere between zero and 572.

The latest proposal, as explained by Reagan and other officials yesterday, expands on the interim plan in three ways:

* First, as Reagan said, "If the Soviet Union agrees to reductions and limits on a global basis, the United States . . . will not offset the entire Soviet global missile deployment through U.S. deployments in Europe. We would, of course, retain the right to deploy missiles elsewhere."

Officials have said this means that if both sides, for example, agreed to 300 missile warheads each and the Soviets decided to keep 100 in Asia and 200 in Europe, the United States probably would deploy only 200, not all 300, in Europe.

Paul H. Nitze, the chief U.S. INF negotiator, has been authorized to discuss levels between 50 and 450 missile warheads for each side, officials said.

Such a proposal would ease concerns of European allies who have privately expressed the view, officials here said, that the United States should not try to balance the combined Soviet missile force in Europe and Asia with U.S. deployments in Europe.

It would also ease Soviet "concerns" that they could face an imbalance in Europe while acknowledging that the Soviets, with a potential enemy in China, feel the need to keep some warheads facing Asia.

The United States, however, would retain the right to deploy as many as 300 warheads but would not necessarily do so, or it could keep the extra ones in this country rather than abroad. Officials said there are no plans to deploy any of the new U.S. missiles in Asia.

The Soviets, however, have steadily maintained that they would be at a disadvantage because of 162 British and French nuclear missiles aimed at them.

The United States, Britain and France reject this, arguing that these are sovereign forces not meant or able to deter a Soviet attack on the rest of Europe and that only U.S. missiles can do so.

This issue has been a major stumbling block since the start of negotiations, and officials said yesterday that the allied position on this point has not changed.

On the other hand, if the United States allows the Soviets a global edge by matching missiles in Europe but allowing the Soviets to keep some in Asia, the West may be able to argue that as a way to balance British and French forces.

* Second is an attempt to be "more flexible" and involves considering "mutually acceptable ways to address the Soviet desire that an agreement should limit aircraft as well as missiles."

The Soviets have continually sought to bargain about U.S. attack jets based in Europe and able to carry atomic bombs. This change, in principle at least, allows that to happen.

But officials stress that this remains a far cry from agreement because NATO maintains that the Soviets have about 2,500 such aircraft compared with NATO's 800. Moscow rejects these figures.

* Third, the president said, is that if the Soviets agreed to global reductions to equal levels, "we are prepared to reduce the number of Pershing II ballistic missiles as well as ground-launched cruise missiles."

The Soviets are believed to be more concerned about the rocket-powered Pershings, which can strike a target inside the Soviet Union with only about 12 minutes of warning, than about slower-flying, jet-powered cruise missiles that might take a few hours to reach their targets.

In tying any new agreement to the need for "effective verification," the president said, "we have negotiated arms agreements, but the high level of Soviet encoding hides the information needed for their verification. A newly discovered radar facility and a new ICBM raise serious concerns about Soviet compliance with agreements already negotiated."

Although some conservative lawmakers have urged Reagan to declare Moscow in violation of previous missile agreements, the president stopped short of doing so.

Reagan also admonished the Soviets for failure to abide by human rights provisions of the 1975 Helsinki agreements on cooperation in Europe and for using "yellow rain and other toxic agents" in Afghanistan and Southeast Asia despite international agreement not to use biological weapons.