House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) will deliver a letter from President Reagan to Soviet leader Mikhail Gorbachev Wednesday elaborating on a U.S. summit proposal, a member of the U.S. congressional delegation visiting here said today.

The delegation member, who asked not to be identified, said that the letter is a follow-up to Reagan's invitation, delivered by Vice President Bush last month after the funeral of Konstantin Chernenko, to hold talks with the new Soviet leader.

In his first major foreign policy statement, delivered in an interview last Sunday with the Soviet Communist Party newspaper Pravda, Gorbachev said he had given a "positive" response to the original invitation. Both Washington and Moscow have said, however, that neither the time nor place for a summit has been discussed substantively.

White House officials confirmed that O'Neill is carrying a letter to Gorbachev in which Reagan "reaffirms" his interest in a summit meeting, Washington Post staff writer David Hoffman reported from Santa Barbara, Calif., where the president is vacationing.

An official said the reaffirmation concerned "the importance the United States attaches to a resolution of the problems between the two countries" but does not suggest a time or place for the meeting. They said the letter was written after Gorbachev's private acceptance of Reagan's invitation.

Administration officials previously have said that there are several possibilities for a summit venue and have suggested the possibility of a meeting at the United Nations this fall. They also have said that a likely outcome of such a meeting would include agreement on one or two items while also serving as a get-acquainted session for the two leaders.

[A senior U.S. official said the timing of a summit may well depend on how quickly Gorbachev will be able to consolidate his power. The official said Gorbachev must deal with pressing economic and energy problems in the near future as well as fill vacancies in the Politburo and that this would most likely preclude a summit with Reagan in the next few months.]

O'Neill, as head of the delegation of 13 members of Congress here under a parliamentary exchange program, will lead a small group of legislators in Wednesday's meeting with Gorbachev. Today, the delegation met with Foreign Minister Andrei Gromyko and with deputies of the Supreme Soviet, or legislature.

The congressmen said they raised concerns over Soviet emigration policies, other human rights issues and the recent fatal shooting of U.S. Army Maj. Arthur D. Nicholson Jr. at a Soviet military installation in East Germany.

Following the session with Gromyko, O'Neill pronounced it "a good discussion. He is a very intelligent man, very impressive. I enjoyed it."

An account of the meeting issued by the Soviet news agency Tass said Gromyko pointed out the "exceptional importance of relations between the U.S.S.R. and U.S.A. as a factor of international politics."

According to several members of the group, the conversations with Gromyko centered on Soviet objections to Reagan's Strategic Defense Initiative, or "Star Wars" program, a common theme among officials here.

The congressmen said Gromyko also raised the issue of a unilateral Soviet freeze on medium-range weapons, announced by Gorbachev on Sunday, but that the subject was not extensively discussed in meetings with the foreign minister nor with other Soviet officials.

The freeze announcement was met with skepticism by the Reagan administration and other western leaders, who described it as an old proposal that, if reciprocated by NATO with a freeze on deployments of cruise and Pershing II missiles in Western Europe -- as Gorbachev suggested -- would leave the Soviets with a wide advantage in European-based nuclear weaponry.

A Soviet source said the U.S. reaction to Gorbachev was greeted with "regret, but no surprise." "We think it is inappropriate if the U.S. government says it wishes to work for better relations . . . to reject these proposals off the cuff," said Vladimir Alexeev, a commentator for the Novosti news agency.

Observers and western diplomats here also have questioned the apparent ambiguity in Gorbachev's statement, saying it is not clear if the freeze applies only to medium-range Soviet nuclear weapons deployed in Europe, or also to deployments east of the Urals in Soviet Asia. Two-thirds of the Soviets' 414 triple-warhead SS20 medium-range missiles are targeted on Europe, and the United States has said those in Asia should also be considered in any accord.

Although discounting the Gorbachev initiative as a propaganda ploy aimed at Western Europe, western diplomats noted that his positive response in principle to a summit and the tone of his Sunday statement were conciliatory, and effectively improved "the atmospherics" of East-West relations.

Today's Tass report on the congressional delegation's meeting with Gromyko said that current arms control talks between the two superpowers in Geneva "open up a good opportunity for reaching agreements. Whether the opportunity will be realized depends on the U.S."

At the three-hour meeting at the Supreme Soviet, the congressmen cited cases of Soviets whose exit visas were said to have been denied, contrary to the Soviet Union's agreement to allow families to reunite across borders.

The congressmen said these cases -- including Soviet Jews wishing to emigrate to Israel and Soviets married to Americans -- were an impediment to improved U.S.-Soviet relations.