By a vote of 9 to 6, the Senate Foreign Relations Committee yesterday rejected the first serious attempt to significantly alter the SALT II treaty with an amendment the Carter administration has described as "a killer."

While the committee proceeded with its line-by-line consideration of the strategic arms limitation treaty, Majority Leater Robert C. Byrd (D-W. Va.) pressed ahead with less visible efforts to achieve new consensus positions on two major SALT issues in hope of building a two-thirds Senate majority for the treaty.

The amendment rejected by the committee was proposed by Sen. Howard H. Baker Jr. (R-Tenn.). It would have forced the Soviet Union to partially count its medium-to-long-range bomber known as Backfire under the treaty's numerical limits on strategic weapons systems. Baker also proposed to count some American medium-to-long-range bombers.

The Backfire, Baker suggested, should count as two-thirds of a strategic system, while the U.S. FB-111 should count as one-half. "The Backfire bomber can reach and destroy and city in the United States," Baker said, and it is "inconceivable" to him that it not be counted under SALT II. s

Opponents of the proposal, including several senators and administration officials who are taking part in the committee markup, responded that the decision to exclude Backfire from the SALT limits was part of a delicately balanced series of compromises on both sides. Tampering with it now, several said, would reopen several key issues and would be unlikely to result in any better deal for the United States.

"If this amendment is adopted that's the end of the treaty as it's now written," Sen. Frank Church (D-Idaho) said. The chairman of the committee recalled earlier testimony that the United States regarded the exclusion of Backfire as acceptable in light of Soviet willingness not to count hundreds of American weapons systems, bases around the Soviet Union and on aircraft carriers, that could be used to attack the Soviet Union.

Sen. Charles Percy (R-Ill.) quoted to Baker earlier testimony by former secretary of state Henry A. Kissinger, who said an amendment forcing the counting of Backfire would freeze the SALT process and perhaps create a crisis in Soviet-American relations.

Baker said "I don't think it matters" if SALT ii is delayed for a year or more by Senate approval of some substantive amendment that requires reopening negotiations. Much worse, hesaid, would be approving a "bad SALT treaty."

The outcome of the SALT debate in the Senate may depend more on Byrd's efforts to build new consensus positions than on the results of the Foreign Relations Committee markup.

Byrd has established two committees of senators to look for a joint position both on increased defense spending and on forcing "deep reductions" of armaments in the next SALT pact that will appeal to a broad spectrum of senatorial opinion.

The idea that the Senate should try to force both the executive branch and the Soviet Union to make substantial reductions in the next round of SALT talks seems to be gaining a growing number of Senate adherents. (SALT II allows both superpowers to add significantly to their nuclear arsenals.)

Sen. Daniel Patrick Moynihan (D-N.Y.), an active proponent of this idea, said yesterday that the White House "has begun to realize that they're not going to get a treaty" unless it includes some insistence on significant arms reductions in the next round.

Moynihan chairs the ad hoc committee Byrd has formed to look into this issue. Moynihan revealed that he has begun negotiations with the White House on possible formulations that would give the Senate a vehicle for abrogating SALT II if it were dissatisfied with the SALT III negotiations.

The second committee formed by Byrd is seeking some way to commit the Senate, at least in principle, to increased defense spending in the future.