Senate Majority leader Robert C. Byrd (D-W. Va.), yesterday announced his support of SALT II and warned colleagues that their vote on the treaty "can have a profound effect on . . . prospects for world peace for many years to come."

The Carter administration and SALT supporters in the Senate have long anticipated Byrd's announcement, which they hope will provide a needed lift to the treaty's fortunes.

SALT supporters were further heartened yesterday by votes in the Foreign Relations Committee rejecting substantial amendments to the treaty that would hav facilitated American monitoring of Soviet strategic programs, but also would have required reopening negotiations with the Soviet Union. These appeared to be the last important amendments with much chance of passage that critics will offer in the committee. All such amendments have been rebuffed so far.

Characteristically, Byrd announced his position with a detailed, 18-page analysis of the benefits he saw in approving the treaty, and the potential dangers if it is rejected. Though the treaty "is far from perfect," he said, he gave a long list of reasons why it should be ratified.

Anwering reporter's questions, the majority leader took an indirect but unmistakable swipe at his Republican colleague, Howard H. Baker Jr. (R-Tenn.), the minority leader and a SALT critic. "I am more than a little concerned at what appears to be . . . positions taken by presidential contenders in the Republican Party against the treaty," Byrd said.

Baker is the only presidential contender in the Senate who has played an active role challenging SALT II. Later, Baker denied to reporters that he was playing politics with the treaty.

Byrd predicted that the SALT debate could begin on the Senate floor just before or just after Thanksgiving, a forecast that other in the Senate regard as optimistic. Before dealing with SALT, byrd said, he hoped the Senate would act on both the new omnibus energy bill and the "windfall profits" tax measure, each of which could easily consume two weeks of floor time in November, and perhaps more.

The majority leader said he saw "no reason whatsover" to let the debate on SALT II slip into next year. He said he would hold the Senate to a Christmas vacation of only about 10 days, in order to get action in this Congress.

Byrd made his statement in favor of the treaty to a crowded press conference in the Capitol's Lyndon B. Johnson Room, just off the Senate floor. The majority leader's wife and numerous members of his staff joined reporters and television cameramen for the occasion. Byrd had edited his 18-page statement into a five-page summary, which he read to the gathering.

"The Senate has an opportunity to help diminish the potential for nuclear destruction, to help make the world a more secure and safer place," Byrd declared. According to associates, he is speaking in similar terms in private, and sees the SALT debate as a truly historic moment in his tenure as majority leader.

In recent weeks Byrd has shown visitors copies of the SALT hearings in the Foreign Relations and Armed Services committees that he has personally annotated and underlined. Yesterday he quoted from many witnesses' testimony on the treaties to bolster his own pro-SALT position.

Byrd endorsed certain modest alterations to the treaty that the Foreign Relations Committee has already adopted, but opposed more substantial amendments.

The Foreign Relations Committee, acting in secret, yesterday voted by unexpectedly large margins not to add substantial changes to the treaty to enhance American ability to monitor Soviet behavior.

As a result, committee sources said, SALT II now appears likely to emerge from the committee "technically unscarred," as one put it.

An amendment offered by John Glenn (D-Ohio), that would have banned the encoding of radio messages sent back to earth from rocket-test flights was defeated 9 to 6. Another Glenn proposal to require the Soviets to give advanced notification every time they planned to test a land-based rocket was defeated 10 to 5.

The committee approved two relatively insignificant reservations yesterday, one urging the administration to negotiate for advance notification of rocket tests in the next SALT agreement, the other cautioning the Soviets that encoding of missile radio messages (known as telemetry) could be the basis of future American protests and even withdrawal from the treaty.

Despite committee votes against his amendments yesterday, Glenn repeated his position that he "hoped" to be able to vote for SALT II on the floor, while warning that he could only do so if he feels the United States has made significant progress in compensating for the loss of its listening posts in Iran.