Senate Majority Leader Robert C. -- the one man who might still be able to orchestrate Senate approval of President Carter's SALT pact -- has already played an extensive but hitherto secret role in backstage political and diplomatic maneuvering surrounding the treaty.

Byrd withheld his own formal endorsement of SALT II until Thursday, but long before that he had been engaged in secret diplomacy with Soviet Union to try to prevent a premature demise of the strategic arms limitation treaty in the Senate. The West Virginia Democrat's direct contact with Soviet officials is an apparently unprecedented initiative by a Senate leader.

This account of Byrd's role was volunteered to The Washington Post. It reflects the majority leader's view of events that other sources confirm did take place. Byrd is presently preparing to play a major role in the SALT debate to try to ensure its passage -- an uphill battle, in Byrd's view.

Byrd's most dramatic personal intervention in SALT-related diplomacy came on Sunday, Sept. 23 when the Soviet ambassador to the United States, Anatolity F. Dobrynin, flew from New York at Byrd's request for a secret meeting in the majority leader's Capitol office.

This was a tense moment in Soviet-American negotiations over the Soviet combat troops in Cuba whose presence had been revealed dramatically by Sen. Frank Church (D-Idaho) on Aug. 30. Preliminary talks between Secretary of State Cyrus Vance and Dobrynin had made no progress, and Vance was about to meet the Soviet foreign minister, Andrei Gromyko, while both were in New York for the United Nations General Assembly. The Carter administration was still hoping that the Vance-Gromyko talks would produce some Soviet concession that would change the "status quo" in Cuba, which Vance and Carter both had said was unacceptable.

Byrd was gloomy about the prospects for these talks. He felt -- and had told Carter -- that the issue had already been blown far out of proportion, and that statements about the status quo being unacceptable were wrongheaded, since the United Santes didn't know what that status really was.

Although Byrd thought the flap over the Soviet troops was a phony issue, he also believed that because of it SALT was hanging by the proverbial thread in the Senate. He blamed the administration for creating this precarious situation, and felt that only by beating an elegant retreat from the mess it had created could the White House save its treaty.

This would require Soviet help, at least indirectly, which is why Byrd wanted to see Dobrynin. After deciding in a meeting with aides that he might be able to play a constructive personal role, the majority leader had called Vance the previous Friday night with a reminder that he had had useful talks with Gromyo and Soviet President Leonid I. Brezhnev on his own trip to the Soviet Union in July. Byrd asked the secretary if it would be useful now for him to see Dobrynin and ask to try to calm the rhetoric from the Soviet side, while pointing out how precarious the treaty's chances had become because of the flap.

After considering this overnight, Vance called back to approve Byrd's proposed initiative. The majority leader then located Dobrynin in New York, where he had gone to meet with his boss, Gromyko. The ambassador agreed to come to Byrd's Capitol office at 11 a.m. Sunday.

Byrd, his foreign policy aide Hoyt Purvis, Dobrynin and one aide sat down around Byrd's mahogany conference table. The senator told Dobrynin he would like to make a statement and did not expect any comment or reaction from the Russian until he was finished. He spoke for about 45 minutes. He said that he had still not made up his own mind on SALT II, but that the treaty would have no chance of approval unless the tense atmosphere created by the Cuban incident could be dissipated. He offered two recommendations:

First, Byrd said, it would be in everyone's best interest to avoid any more heated rhetoric about the Cuban issue. Second, it would be useful if the Soviet side could carefully evaluate its position with an eye toward some gesture that could help Carter out of the jam.

Byrd indicated to Dobrynin that he felt American position in this flap was less than compelling. Nevertheless, he said, the political furor set off by Church's announcement that SALT II could not be approved until the Soviet troops were withdrawn had gravely endangered the treaty's prospects. Any positive gesture the Soviet side could make would be very helpful.

Byrd told Dobrynin that the idea for this meeting was entirely his own, and that his objectives were to try to improve the atmosphere for a debate on SALT on its merits, uncolored by the emotions surrounding the Cuban matter.

When the Soviet ambassador left Byrd called Vance and said he would like to meet that night with the president. There was a party scheduled at the White House, but sometime after 9 p.m. Byrd sat down with Carter, Vance and Zibigniew Brzezinski, Carter's national security adviser. Byrd repeated the message he had delivered to Dobrynin. Carter asked for Byrd's suggestions, and he gave three:

First, Byrd said, the United States should cool its rhetoric on the Cuban issue and try to let quiet diplomacy work. He said newspaper stories were unhelpful in suggesting harsh retaliations the White House was considering if the Soviets did not alter the status quo. He declared frankly that the president did not have a very strong case.

Byrd said it was inappropriate for a mighty nation to go into a delirium over about 2,300 Soviet troops that had neither airlift nor sealift capability to leave Cuba.

Second, said Byrd, there should be no more White House meetings with congressional leaders to discuss the Cuban situations. Byrd had personally boycotted such a meeting on Sept. 20 after advising Vance that it could only contribute to the appearance of a crisis at hand -- an appearance Byrd felt was unjustified.

Third, Byrd advised, do what you can to get this flap behind us. In his opinion, the Soviets would not withdraw the troops because they had been in Cuba for many years, and because Moscow thought the entire dispute was a political incident of no military significance. If they withdrew now, other countries would have to conclude that the Soviets had backed down.

Afterward Byrd felt his intervention might have done some good. He was pleased that subsequent Soviet public statements were relatively restrained, and that the Soviets did produce some last-minute "assurances" to Carter that helped the president cover his retreat from the "unacceptable status quo" stance. Byrd was pleased with Carter's final speech to the nation on the issue, which he felt helped provide realistic perspective.

The Carter administration and numerous pro-SALT senators are hoping Byrd can find a way to revive pro-treaty sentiment and win Senate approval. The majority leader appears eager to make this effort, though privately and publicly he refuses to make any predictions of success.

Byrd has already organized special committee of senators interested in defense spending and future arms reductions, both issues that could bear on some senators' final decisions. Byrd hopes to build a broad consensus on these issues that will help the treaty.

Before the Cuban flap, Byrd believes, SALT II was all but assured of Senate approval. Now the outcome remains problematical.