Bowing to the insistance of key uncommitted senators, the Carter administration has apparently decided to promote its SALT treaty by giving the Senate an early look at its next defense budget and revised five-year defense plan.

The administration will advance the budgetary calendar by two months to make this possible. This decision is a key element in the administration's revised strategy for selling SALT II to the Senate, a strategy whose outlines began to emerge here yesterday.

The decision on the five-year defense plan means that, in effect, the administration will be presenting the outlines of a defense budget independent of its plans for domestic spending, which will be published at the usual time in January. The Office of Management and Budget fought this idea, but President Carter indicated yesterday that he had acquiesced for the sake of SALT.

At a White House briefing for prominent citizens, Carter noted that Senate Majority Leader Robert C. Byrd (D-W.Va.) had said the military budget would be released in November, and joked that "our strategy has to be modified to accommodate things that Sen. Byrd says . . . It's highly likely," Carter went on, "that we will develop the military portion of our budget early enough" to give the Senate a good idea of its contents before voting on SALT II.

Sen. Sam Nunn (D-Ga.), who may hold the key to Senate approval of SALT II, has said he will not vote for the treaty until he sees a new defense budget for next year and a five-year defense plan, and is satisfied they include enough growth to improve the country's defenses. White House officials acknowledge that Nunn must be satisfied if SALT II is to win two-thirds Senate approval.

A number of uncommitted senators say they will take a lead from Nunn when casting their votes on the treaty.

Another element in the administration's new SALT-selling strategy is to accept some kind of Senate reservation to the SALT treaty that would require certification from the president that the Soviet "combat brigade" in Cuba poses no threat elswhere in the Western Hemisphere.

At the White House briefing yesterday, Carter said something along these lines could be worked out with the Senate. On the NBC "Today" show yesterday, Secretary of State Cyrus R. Vance was more explicit, saying "I do not rule out some form of certification" by the president regarding the Soviet troops in Cuba.

Other knowledgeable sources said the administration is hoping to work out a reservation with Sen. Frank Church (D-Idaho), chairman of the Foreign Relations Committee, who has made a big issue of the Soviet troops in Cuba. The reservation might say something to the effect that the SALT treaty could go into effect only when the president certifies that Soviet troops in Cuba are not equipped to allow them to threaten any other country in the Western Hemisphere.

Critics of SALT and the president's handling of the Soviet troops flap have already dismissed such a gambit as "a copout," in the words of Senate Minority Leader Howard H. Baker Jr. (R-Tenn.). The administration hopes that most senators would accept such a reservation as an adequate response to the Soviet troops in Cuba.

Carter outlined the rhetorical elements of the administration's revised SALT-selling strategy at the briefing for the citizens, mostly from Ohio, Oklahoma and Virginia.

In essence, the president said, it is an "or else" presentation -- either the Senate gives its approval to SALT II, or else the United States faces a number of dire consequences. Carter listed these:

Defeat of SALT II "might be an irretrievable mistake" because it would interrupt "seriously and perhaps permanently" the efforts of a succession of American governments to negotiate reductions or controls on nuclear arms.

Without SALT II the Soviets could freely conceal their weapons programs, testing and so forth (deliberate concealment would be banned by the treaty), "and every time we heard a rumor about what the Soviets were doing . . . we would probably have to assume the worst." This would intensify the arms race, the president said.

The confidence the NATO allies have placed in the United States as leader of the alliance on military matters "would be severely damaged, perhaps destroyed," if the Senate rejects the arms treaty. In that case, Carter said, the United States could not expect the allies to join in a cooperative policy with America to modernize nuclear weapons in Europe or other NATO forces.

Defeat of the treaty would encourage third world countries that have the necessary industrial capacity to develop nuclear weapons, and the United States would be helpless to prevent this. "It would be embarrassing for me" to ask a third world leader not to develop nuclear weapons if SALT failed, Carter said. The reply would be "how can you dare" raise the issue if you yourselves cannot limit your own weapons, he predicted.

Senate rejection of SALT II "would hand the Soviet Union the most valuable propaganda weapon that they could possibly dream of," allowing them to claim the role of frustrated peacemakers, thwarted by American intransigence.