The Carter administration opened its formal defense of the strategic arms limitation treaty before the Senate yesterday, arguing that a combination of accelerated defense spending and the treaty is needed to preserve American security.

In opening hearings before the Senate Foreign Relations Committee in the great marble vault of the Senate Caucus Room, Secretary of State Cyrus R. Vance and Defense Secretary Harold Brown adopted a bare-bones defense of SALT II. By itself, both said, the treaty will not guarantee "essential equivalence" with the Soviet Union; billions more must be spent on new weapons as well.

But without the treaty, both said, the task of protecting American national interests would become more expensive and more dangerous.

Vance urged the senators not to succumb to a temptation to amend the new treaty, at least until they had heard all the administration's arguments for it. But Vance acknowledged that senators have the right to change the agreement, and that "President Carter will have to accept" whatever the Senate does.

In their joint appearance the two secretaries never mentioned detente with the Soviet Union, nor did they predict any secondary benefits in improved Soviet-American relations if SALT were approved.

This was the first day of the great SALT debate, and the old Caucus Room - made famous most recently by the Watergate hearings - was filled with television cameras, reporters and spectators. But there were only occasional flashes of drama and few attempts to seriously challenge either the treaty or the two secretaries who testified for it.

Sen. Howard H. Baker Jr. (R-Tenn.), who has established himself as an opponent of the treaty as it now is written, injected the first note of political excitement into the generally stolid hearing with a forceful speech in "deadly" Soviet land-based missiles, implying that Soviet advantages in this weapons category were dangerous to the United States.

Defense Secretary Brown responded to Baker in kind, saying with some emotion that singling out one category of the strategic equation was misleading. "You've picked probably the least significant criterion," Brown told Baker.

Later, the chairman of Baker's presidential campaign, Richard G. Lugar (R-Ind.), pursued the same line of questioning, asking Brown to confirm that the Soviets' SS18 supermissile was the one weapon in the Soviet arsenal that created grave problems for the United States - a position Baker has taken publicly.

Brown gave Lugar a lecture on the arithmetic of nuclear strategy, arguing that the theooretical vulnerability of American land-based missiles to a sneak attack by Soviet land-based missiles would exist even if the Soviets dismantled all their SS18s. Lugar did not dispute Brown's point but went on to another line of questioning.

If any new ground was broken yesterday, it was in Vance's answers to questions from Sen. Jacob Javits (R-N.Y.) about the Carter administration's attitude toward changing the treaty. Why, Javits asked, had President Carter jointed with Soviet authorities in warning that changes would be unacceptable?

Carter spoke before the Soviet leaders said anything, Vance replied. Then he went on to soften the administration's position appreciably:

"We recognize the Senate's power to advise and consent includes the power" to recommend changes, Vance said, but he added that efforts should be made to alter the pact "only in the case of clear and urgent need." The administration would have to evaluate any proposed changes in terms of whether they served U.S. interests, whether they would require reopening negotiations and whether they would lead to "no treaty at all," Vance said.

These carefully chosen words seemed that has developed between the Senate and the White House on the subject of changing the treeaty.

Numerous senators, their aides and some administration officials have said privately in recent days that some Senate changes in the treaty are inevitable.

In their prepared testimony, Vance and Brown returned again and again to the point that the United States must modernize strategic forces to maintain a stable balance with the Soviet Union. "The security of the United States requires us to maintain . . . forces that are equivalent to those of the Soviet Union," as Vance put it.

This emphasis reflects a decision inside the adminstration that "equality" is the most important issue in the SALT debate, and that the administration must convince both the Senate and the nation that it is committed to maintaining equality.

Brown gave detailed figures on what his version equality would cost - an additional 25 percent on the strategic arms budget, for starters.

Under SALT II, U.S. strategic spending should rise from about $10 billion a year to around $12.5 billion, Brown said. If SALT II is rejected, he said, that figure should rise to $15- $16 billion.

One new member of the administration's SALT-selling team, Washington lawyer Lloyd N. Cutler, who argued in favor of emphasizing the quality issue in yesterday's testimony, sat throughout just behind Vance and Brown. Several times Cutler leaned forward to whisper suggestions, and on several occasions his advice appeared to be taken, leading to further comments from the secretaries.

Carter has named Cutler to an unpaid position as coordinator of the administration's case on behalf of the treaty.

Also appearing with Vance and Brown was Ralph Earle, chief U.S. negotiator for the treaty. He was seated to Vance's right.

In his prepared testimony. Vance described four "imperatives of our national security" all of which would be served by approving SALT II. First, he said, the treaty "will greatly assist us in maintaining a stable balance of nuclear forces" by inhibiting the Soviet strategic program while allowing the United States to conduct an extensive modernization program of its forces. ("SALT II constrains them more than it constrains us," Brown said later.)

Second, Vance said, the treaty would improve "our ability to monitor and evaluate Soviet strategic forces and programs." The treaty includes important provisions prohibiting deception and concealment, Vance said. It also bans the Soviet mobile missile SS16, which might be difficult to monitor, he added. "We will be able to detect any Soviet violations before they could affect the strategic balance," Vance said.

Brown was asked about earlier statements he had made that it would take a year to restore SALT monitoring capability lost to the United States when its listening posts in Iran were closed early this year. Brown reiterated Vance's comment that the Soviets could not get away with any significant cheating.

The third argument for SALT II, Vance said, is that it would make SALT III possible. He acknowledged disappointment that this pact did not go further, but added: "We should build on the progress we have made. The alternative is to return to an unrestrained arms competition. . . ."

Fourth, Vance went on, America's North Atlantic Treaty Organization allies have a keen interest in seeing SALT II ratified: "Defeat of the treaty would be a profound blow to our closest friends. Its approval will benefit our most valued alliances. It will signal continued American leadership for peace."

Both Vance and Brown offered grim pictures of a nuclear war. This was Vance's:

"Together, the arsenals of the United States and the Soviet Union already hold more than 14,000 strategic nuclear warheads and bombs. The smallest of these are several times as powerful as the bomb that destroyed Hiroshima. If a fraction of those weapons were ever fired, tens of millions of our people and tens of millions of the Soviet people would perish. Nuclear war would be a catastrophe beyond our imagination - for the aggressor as much as the victim."

In his prepared remarks Brown said recent trends in strategic forces favored the Soviets, but he also predicted that Soviet forces would grow much faster without SALT II than with it. Defeat of the treaty could cost the United States an additional $30 billion to keep up with the Soviets over the next decade, he claimed, without elaborating on how that would be spent.

If SALT II were rejected, Brown said, "it is nor certain" that the United States could establish a better position relative to the Soviets than with the treaty.

Talk of more defense spending with SALT upset one member of the Foreign Relations Committee, George McGovern (D.-S.D.).

"True security is attainable only by mutual and substantial arms reduction," McGovern said. "Piling on more nuclear overkill does not enhance our diplomatic prestige or our military defense. If the Soviets wish to bankrupt themselves on weapons programs, let them do it. . . ."

Warning that his vote for SALT II is not certain, McGovern said: "I am not willing to endorse an arms control hoax which turns a weapons limitation treaty into nuclear expansion which is militarily unwarranted, fiscally wasteful and diplomatically destabilizing."

Several senators expressed concern that the protocol to SALT II would prevent the United States from sharing military technology with the other NATO members. Vance and Brown flatly rejected this view. Under SALT II, Brown said, America even could provide its allies with Trident missile-carrying submarines if it wanted to. The Trident is one of the country's most potent strategic weapons.

Of the senators on the committee, only Baker chose to fire across the administration's bow on the first day of hearings. He said the issue was whether "we gave away the store" in negotiating SALT II.

When the SALT process began a dozen years ago Baker said, the United States enjoyed clear Military superiority, which has now been lost. He suggested that this left the country in a seriously weakened position.

Joseph R. Biden Jr. (D-Del.) followed Baker in questioning yesterday, and hinted at the debate tocome by ridiculing Baker's analysis of declining S.U. power.

"I don't recall us in 1967 being able to work our will" in the world, Biden said sarcastically. U.S. influence, he said, depends on "the morality of our cause . . . the strength of our economy . . . and our conventional forces."

Frank Church (D-Idaho), the committee chairman, noted that many Americans may be listening to the SALT hearings this month while waiting in gasoline lines. He urged the public to recognize that this debate touches their personal interests.

Edward Zorinsky (D-Neb.) said he discovered while home over the Fourth of July holiday that many of his constituents don't believe the government will do what is necessary to maintain American strength on an equal footing with the Soviets. "What kind of commitment can we unilaterally make to the American public?" Zorinsky asked. CAPTION: Picture, For the affirmative: Earle, Vance and Brown . . . By James K. W. Atherton - The Washington Post; Picture 2, . . . and for the negative: committee member Baker By James K. W. Atherton - The Washington Post; Picture 3, Vance and Brown in Caucus Room: SALT alone is not enough; more defense spending is needed as well. By James K. W. Atherton - The Washington Post