I realize that personal memoirs are a far cry from objective history. But in describing my role in the "discovery" of the Russian combat brigade in Cuba and subsequent developments concerning the SALT II treaty, former president Jimmy Carter strays so far from the facts that I feel impelled to publish a reply.

In his book, "Keeping Faith," Carter charges me with having divulged the "discovery" after reading of it in a confidential daily intelligence report. He ascribes my motive to that of fending off a conservative onslaught in Idaho, and concludes that I was "absolutely irresponsible" in stating that "unless the Soviets totally withdrew the troops, SALT could not pass."

The truth is that I first learned about the brigade's existence on Aug. 30, 1979, not from any intelligence source, but in a telephone call from David Newsom, Carter's No. 3 man at the State Department. Newsom said he wanted me advised before "the news surfaces in the press."

I immediately called Secretary of State Cyrus Vance, who confirmed Newsom's message, and, in response to an inquiry of mine, said that the administration did not plan to announce the "discovery." I replied that someone in a position of responsibility should make the announcement, and that if neither he nor the president intended to do it, I would. Vance did not object, but merely cautioned me against accusing the Russians of violating any agreement with the United States respecting Cuba. He said these agreements had been reviewed, and they contained no reference to combat troops.

Admittedly, I should have anticipated that my reason for making the announcement would not be believed; that the press, far from treating me as a "favorite," to use Carter's term, would subscribe to the White House line that my motive was basely political. I had been in public life long enough to know that the bearer of bad tidings usually gets blamed for them.

As for my allegedly "irresponsible" statement that the Senate would not ratify the SALT II Treaty while Russian combat troops remained in Cuba, it was made in straightforward recognition of a problem we had to face. It was apparent that a number of senators would use the presence of the Russian combat brigade in Cuba as a reason for voting against the treaty, a plain fact which, to my knowledge, nobody has ever denied.

In an effort to surmount this hurdle, I proposed a reservation that, before the treaty took effect, the president should first certify that Soviet troops in Cuba were no longer engaged in a "combat role." Soviet compliance with such a requirement did not appear overly onerous, since the Russians had never admitted that combat forces were stationed in Cuba in the first place. The reservation was adopted by the Senate Foreign Relations Committee on a vote of 13 to 2, which helped clear the way for the committee's approval of the treaty itself on Nov. 9, by a vote of 9 to 6. Ten days later, the committee's report backing the treaty was published, and Majority Leader Robert Byrd, pronounced it the best he had ever read.

Then, on Dec. 20, the Senate Armed Services Committee, in an unprecedented move, adopted a report of its own, asserting that the SALT II Treaty, "as it now stands, is not in the national security interests of the United States."

In the aftermath of this devastating blow, one which Carter fails even to mention in his book, we could never count more than 50 votes in the Senate for the treaty. The Democratic leadership was, quite properly, reluctant to call it up for debate while it remained so far short of the 67 votes needed for ratification. As we were probing for these additional votes, the Russians suddenly invaded Afghanistan, and the treaty, at Carter's own request, was shelved.

As a proponent of SALT II, I was keenly disappointed at the outcome. But recriminations over the treaty's death are, at the very least, premature. SALT II still lives. The United States has pledged to abide by its provisions, as long as the Russians do likewise, a commitment reaffirmed by President Reagan, even though, as a candidate, he campaigned against the treaty. This commitment, in which the Soviet Union has joined, differs very little from what would have applied had SALT II been formally ratified.

One day, someone who can investigate the matter with detachment will write the full story about the episode of the Russian combat brigade. It will then be revealed that many mistakes were made by nearly everyone concerned. However, my purpose here is not to accuse others or apportion blame. For those of us who were involved, it is time to sheathe the long knives.

The writer was formerly a Democratic senator from Idaho and chairman of the Senate Foreign Relations Committee.