Moscow's reaction to the planned December deployment of Pershing II and cruise missiles in western Europe is expected to be shorter and "less bombastic" than once anticipated, according to key Reagan administration officials.

Although Soviet negotiators in Geneva have told their American counterparts that sessions on intermediate-range nuclear forces (INF) and strategic arms reduction talks (START) will be suspended in the wake of any deployment, officials here do not expect the walkout to be permanent. A few believe it will not last longer than a few months.

"It will be pro forma," one official said.

The Soviet government, another official said, is on the defensive and having unexpected problems trying to decide how to respond.

It cannot appear to "condone the American deployments" by accepting any compromise at the INF talks, one source said. On the other hand, this official added, Soviet officials cannot afford to disregard the anti-nuclear movements in eastern and western Europe by spurning the negotiations and adding more nuclear missiles of their own.

American officials who see the Soviets as trapped in an unprecedented political dilemma, sources said, are primarily the same ones who don't want to make things any easier for Moscow by further modification of the current U.S. position at the INF talks.

This view of the Soviet position is vastly different from that given to American congressmen in Moscow recently. Rep. Thomas J. Downey (D-N.Y.) said last week that Soviet officials had impressed on him the idea that American deployment will mean another serious round in the nuclear arms race.

Some American specialists in Soviet affairs have warned the administration that although Kremlin political leaders may unhappily accept U.S. deployment, Soviet military leaders may not.

Long considered masters of political propaganda, the Soviets have had a series of public and private failures over the past few months on the missile deployment issue.

The first, and most sources say the most important, Soviet misstep was the heavy-handed public support given to the Social Democrats in West Germany's March parliamentary elections. The victory of the Christian Democrats, whose leaders forcefully supported the American missile deployment, was a major Moscow setback.

Moscow then compounded its difficulties by publicly threatening to place additional nuclear missiles in East Germany and other Warsaw Pact countries. That statement not only failed to frighten the West Germans, who already faced more than 1,000 Russian nuclear warheads, but as one American official pointed out, "the East Europeans don't like the idea either."

At a Moscow meeting last month, Romanian President Nicolae Ceausescu refused to sign the closing statement, and Moscow's other allies failed to embrace publicly the idea of placing new Soviet nuclear weapons in their countries. Hungary in particular is reportedly unhappy about having to take in nuclear weapons, according to administration officials.

Moscow's position that it would not limit the number of its SS20 missiles east of the Urals, and its statement that it could move missiles from the European sector of the Soviet Union into the far eastern sector has created an outcry in Japan.

Meanwhile, U.S. officials have been hard pressed to determine how the Soviets could carry out their oft-repeated threat to deploy missiles that will threaten the U.S. mainland in the same way that the Pershing II does with its 10-minute flight time to a Soviet target.

Pentagon experts have noted construction for additional SS20 sites in the eastern Soviet Union that would put targets in Alaska and the northwest United States within range. But, as Reagan officials point out, these hardly represent a serious new threat to this country.