SALT II passed across its first political barrier yesterday when the Senate Foreign Relations Committee voted 9 to 6 to approve it for consideration by the Senate.

The strategic arms limitation treaty with the Soviet Union is unlikely to reach the floor before the end of this month at the earliest, and a long debate is anticipated. Supporters spoke optimistically yesterday, but a careful count of the Senate shows that SALT II is still in trouble, and only an extremely successful political campaign can win the 67 votes needed for its ultimate approval.

The Foreign Relations Committee made 23 changes in the treaty by amending the resolution of ratification on which the Senate will eventually vote. Two of the changes will require explicit Soviet approval, and the others either state American interpretations of treaty provisions or make technical alternations.

Seven Democrats and two Republicans voted for the treaty, four Republicans and two Democrats against. The margin was smaller than the two-thirds eventually needed to approve the treaty on the floor.

All attempts to make substantive changes were beaten back, sometimes by a single vote. Treaty critics led by Howard H. Baker Jr. (R-Tenn.) predicted that the Senate will reconsider those votes and may reverse them.

Baker said the Senate "probably will insist on amendments" to SALT II, and if they are not approved, he predicted, "the Senate will defeat it."

Administration lobbyists hope that they will have more than 50 Senate votes -- a majority -- to beat back all substantive amendments of the sort that would require re negotiation with the Soviet Union. Treaty supporters have come to call these "killer amendments."

But as Baker and other have repeatedly said, if all amendments are beaten, there may not be 67 votes for approval of the treaty. To emphasize that point yesterday, one of the nine senators who voted for the treaty in the committee, Edward Zorinsky (D-Neb.), indicated he would withhold final approval unless more amendments are adopted by the Senate.

"In no way will my vote today predestine my final vote," Zorinsky declared. To win his vote in the end, the treaty must be further altered "in some way and in some manner" to weight it more favorably for the United States.

A high administration official said yesterday that perhaps 15 other senators might end up in a position close to Zorinsky's, raising the prospect of further changes that this official described as dangerous. Without those 15 or so votes, however, SALT II has no change of final approval.

Zorinsky, a little-known first-term Democrat, typifies the senators the White House or Majority Leader Robert C. Byrd (D-W.Va.) will have to win over in the debate. The key swing senator are, with a few exceptions, junior members without big reputations.

The most important exception is probably Sam Nunn (D-Ga.), who has withheld his support for SALT II until he sees and approves the administration's 1981 defense budget and its revised five-year defense plan.Nunn will see those projections within the next two weeks.

All optimistic predictions about SALT depend on the assumption that Nunn will support it.

In recent days President Carter has conferred one-on-one with senators whose votes could be crucial but are still in doubt. The president has seen Henry Bellmon (R-Okla.), Thad Cochran (R-Miss.), J. James Exon (D-Neb.), Howell Heflin (D-Ala.), Daniel Patrick Moynihan (D-N.Y.), Nunn, William V. Roth Jr. (R-Del.) and Alan K. Simpson (R-Wyo.). Typically these private sessions have lasted 45 to 60 minutes, and Carter plans more.

At yesterday's final markup session, most of the Foreign Relations Committee members made statement explaining their votes. The statement revealed both the wide variety of senatorial concerns about the treaty, and general disappointment among both supporters and opponents that SALT II will have such a small impact on the continuing buildup of both Soviet and American strategic arms.

The nine senators who voted for the treaty were Chairman Frank Church (D-Idaho), Joseph Bidden Jr. (D-Del.), Jacob Javits (R-N.Y.), George McGovern (D-S.D.), Edmund Muskie (D-Maine) Claiborne Pell (D-R.I., Charles H. Percy (R-Ill), Paul Sarbanes (D-Md.) and Zorinsky.

The six opposed were Baker, John Glenn (D-Ohio), S. I. Hayakawa (R-Calif.), Jesse Helms (R-N.C.), Richard G. Lugar (R-Ind) and Richard Stone (D-Fla.).

The four Republicans who voted no had previously indicated their positions. So had Glenn, who has explained that he hopes to vote yes on final passage, but will only do so if the United States has new eavesdropping technology in place that compensates for electronic spy stations in Iran that were closed this year. Stone explained his vote by saying "the treaty in its present form should not be ratified" because it is imbalanced in the Soviets' favor.

Apparently the charge that SALT II is tiled toward the Soviets will be the central argument in the debate. All the critics made that point yesterday. Responding, Muskie asked what the critics would be prepared to give up in return for the changes they want to regotiate. He has heard no suggestions, Muskie said.

The committee amendments that the Soviet will have to approve incorporate into the treaty assurances Moscow has given on the range and production of its controversial Backfire bomber, and "joint statements and common understandings" that accompany the text to spell out crucial details.