West German Foreign Minister Hans-Dietrich Genscher travels to Moscow Sunday on a trip viewed here as an important first step toward thawing the long freeze in Bonn's relations with the Soviet Union.

Genscher has not visited Moscow since March 1985, when he received a cool welcome. He expects to have his first lengthy talk ever with Soviet leader Mikhail Gorbachev and to press for progress on disarmament and other East-West issues, during his two-night stay, officials said.

The government of Chancellor Helmut Kohl has long been eager to improve its relations with the Soviet Union, but Moscow has kept the West Germans at arm's length.

Moscow seems to have shifted its tactics, at least for the moment, because of its recently stepped-up effort to woo West European governments in general, the sources said. The Soviets hope to enlist the West Europeans to pressure the United States in nuclear arms talks and other security matters.

In addition, the Soviets may have concluded from the victory of Kohl's center-right coalition in last month's Lower Saxony state elections that the coalition will win next January's national vote as well. The Soviets may have decided that it was best to begin cultivating the Bonn government now, instead of waiting in hope that the left-of-center Social Democrats would come to power, the sources said.

"The decisive factor may have been the Lower Saxony elections, and the dropping of legal proceedings against Kohl" in a campaign financing scandal, a senior western diplomat here said. "The Soviets may have decided that Kohl would win the national election, and that there wasn't any point in adopting policies designed to alienate him."

The Soviet policy of isolating West Germany is thought to have been aimed largely at punishing Kohl for the 1983 decision to allow deployment of U.S. medium-range nuclear missiles.

Soviet officials repeatedly have cited Bonn's security policies as the main factor inhibiting closer relations.

Because of these factors, a warming of relations with the Soviet Union is likely to be a slow process, diplomats and West German officials said.

The sources predicted that the Soviets would not permit Kohl to visit Moscow before the January elections here. Two government sources said that the Kremlin would not give Kohl such a "gift" before the vote.

Government officials and diplomats here, citing evidence of a shift in Soviet attitudes, pointed to visits to Bonn this month by Soviet strategic arms negotiator Viktor Karpov and chemical weapons negotiator Viktor Issraelyan. The sources also noted that the Soviets appear willing to sign a scientific and technological framework accord with Bonn while Genscher is in Moscow. The two sides still were haggling today, however, over clauses in the accord regarding the status of West Berlin.