The Senate Foreign Relations Committee began final consideration of SALT II yesterday, and immediately demonstrated that the treaty's passage through the panel will be a precarious voyage.

Within two hours of the opening gavel, senators wer toying with a new "understanding" on the Backfire bomber that would push the Soviet Union farther than it was willing to go when the treaty was negotiated. That matter was temporarily withdrawn from consideration when the discussion ranged beyond foreseen limits.

The committee did approve two reservations, a formal one that incorporates into the document the joint statements and common understandings that accompany the treaty text, and another that has no effect.

But this first session was just "a warmup," as one committee source put it. The process of marking up the treaty, or considering its provisions one at a time, will take at least two weeks, and probably more.

Sen. Frank Church (D-Idaho), chairman of the committee, began the session in a crowded hearing room, made bright and hot by television lights, with brief remarks comparing SALT II's significance to that of the Treaty of Versailles. The ranking Republican, Jacob K. Javits, (N.Y.), called SALT "critically important . . . to the peace of the world," and added, "the eyes of the world are fixed upon us."

During the hearing, Sen. Charles Percy (R.Ill.) announced that he had received a letter from Gen. Bernard W. Rogers, supreme allied commander in Europe, endorsing the arms limitation treaty and reporting strong NATO support for it.

In July, anti-treaty sources in the Senate said Rogers was skeptical of SALT II, and might have prevented the Joint Chiefs of Staff from endorsing it unanimously had he not been moved in June from his post as Army chief of staff to the NATO command.

Rogers' letter to Percy, however, contained a strong personal endorsement of the treaty, as well as the observation that "ratification of SALT II is considered crucial by our NATO allies."

This is a line on which administration spokesemen are relying with increasing emphasis in their efforts to sell SALT II to the Senate. Defense Secretary Harold Brown said on the NBC "Today" show yesterday that he foresaw "a very serious danger" that NATO might unravel if SALT is defeated.

Rogers said he knew some treaty critics had argued in private, at least, that many European military men and political leaders opposed to SALT II."I do not believe this is the case," the NATO commander wrote.

During deliberations on the treaty, the Foreign Relations Committee plans to consider reservations or understandings to the treaty in three categories: those the Soviets must accept when (and if) the treaty is formally ratified; those that will be brought to the Soviets' attention, but do not require their acceptance, and those that affect intergovernmental relations only on the U.S. side.

The committee leadership, which desgined these categories, opposes any amendment to the treaty text, and so far no member of the committee has formally proposed one. But that is still possible. Treaty opponents on the committee have not signaled whether they intend to offer a lot of substantive amendments at the committee level, or plan instead to propose them on the floor.

SALT II is likely to be approved in the committee by a decisive majority, but what reservations or understandings may be attached remains open. Yesterday's debate on the Soviet Backfire bomber showed why.

Church and Javits proposed incorporating into the Senate's resolution of ratification a reservation that would give binding legal status to the written and oral commitments the Soviets gave in June in Vienna. At that time they proposed to restrict production of the medium-to-long-range Backfire to 30 planes a year.

In the discussion of this, Sen. John Glenn (D-Ohio) read from a letter from the State Department a description of President Carter's statement to the Soviet side in Vienna that he would regard any "significant upgrading" of the Backfire's capabilities as a violation of their assurances.

The Soviets replied that they would not be bound by unilateral American statements.