President Carter laid out his case for a new strategic arms limitation treaty (SALT) with the Soviet Union today, warning that rejection of the pact will lead to "a more perilous world" and "drift into a nightmare of unrestrained arms competition."

In a speech to the American Newspaper Publishers Association here, the president strongly affirmed the ability of the United States to verify the term of the SALT II treaty, now nearing completion after years of negotiation.

Responding for the first time to crities who have charged that loss of intelligence-gatering posts in Iran has crippled U.S. monitoring capability, Carter said "the treaty must-and will be-verifiable from the day it is signed."

Carter did not say explicitly what he means by the term "verifiable," but he edged toward a definition by expressing confidence that "no significant violation" of the treaty can take place without U.S. detection. He suggested more detailed criteria at another point by declaring that "there is no question that any cheating which might affect our national security would be discovered in time for us to respond fully."

The somber address marked the beginning of the president's personal campaigning for Senate approval of the SALT II accord-a campaign that is expected to dominate the political scene in coming months and possibly spill over into the 1980 presidential primaries.

As late as a week ago, White House officials had hoped that basic agreement on the SALT II pact would be reached by the time of Carter's kick-off speech today. Last-minute hitches have delayed the conclusion, which is now said to be unlikely this week.

In Washington, Secretary of State Cyrus R. Vance met Soviet Ambassador Anatoliy F. Dobrynin at the State Department for the 20th time since Jan. 1. State Department sources siad another meeting is expected about Friday, but the sources refused to predict when the tready will be finished.

In his speech here, Carter argued that the SALT II agreement has overwhelming popular support and the backing of U.S. allies. He said it will enable the United State to counter better the Soviet military buildup but will not affect other aspects of American policy toward the Soviet Union, including "our ability to promote our interests and to answer Soviet threats to those interest."

But most of all, the president said, SALT II is necessary because the allternatives are so bleak, including sharply rising defense spending, "and enormous further buildup" of Soviet strategic forces and a spread of nuclear weapons to other nations.

"Without SALT, the world would be forced to conclude that America had chosen confrontation rather than cooperation," Carter said.

"This is the inescapable choice we face," he continued, "for the fact is that the alternative to this treaty is not a perfect agreement drafted unilaterally by the United States in which we gain everything and the Soviets nothing. The alternative, now and in the foreseeable future, is no agreement at all,"

White House officials billed the speech as the president's "most comprehensive" statement on the SALT II negotiations. One aide called the speech "more of a document," to be drawn on by the administration during the campaign to gain Senate approval of the treaty.

"it's dull, in the sense that the actual, technical details of SALT are dull," he said. "It's an attempt to establish a rationale for the treaty before we start running around the country yelling about war and peace, which is the real issue."

No aspect of the treaty will be more crucial to the Senate's decision than the question of verification. This is particulary true since the revolution in Iran and the loss of American intelligence-gathering facilities that were used to monitor Soviet military activities.

Last week, the administration found itself on the defensive on this issue. denying reports that Central Intelligence Agency Director Stanfield Turner had told senators that it will take up to five years to make up completely for the loss of those intelligence assets.

Today, Carter took the offensive on the verification issue.

He argued by their size and nature strategic weapon systems are relatively easy to monitor. In unusually direct words he said our photoreconnaisswords, he said, "Our photoreconnaissance satellites survey the Soviet Union on a regular basis and give us high confidence that we will be able to count accurately the numbers of al these systems."

He added that through satellites and other means the United States "can determine not only how many systems there are, but what they can do."

The United States is concerned with the loss in Iran, but those stations, were among many intelligence sources and must be kept "in perspective," he said. This loss of monitoring capability relates "principally" to that part of the SALT II treaty limiting the modernization of land-based ballistic missiles, Carter said.

"The bottom line," the president added, "is that if there is an attempt to cheat on the SALT agreement-including the limits on modernizing ICBMs-we will detect it, and we will do so in time fully to protect our security."

Carter's audience of publishers sat soberly through the 30-minute speech and responded with polite applause.

The president made no attempt to rouse his audience with his message, as he spoke in his soft, southern drawl.

"Each generation of Americans faces a choice that defines its character-a choice that is also important for what it says about our nation's outlook on the world," he said. "In the coming months, we will almost certainly be faced with such a choice: whether to accept or to reject a new strategic arms limitation treaty. The decision we make will profoundly affect our lives-and the lives of people all over the world-for years to come."

The United States and the Soviet Union, Carter said, "have a common interest in survival and we share a common recognition that our survival depends, in a real sense, on each other. The very competition between us makes it imperative that we bring under control its most dangerous aspect-the nuclear arms race."

The president argued that the arms treaty, coupled with the increases in defense spending that he has recommended to Congress, offers the United States the best chance of countering the dangers of the Soviet military buildup.

"What causes us concern is not the curren [Soviet-American] balance, but the momentum of the Soviet strategic buildup," he said. "Over the past decade, the Soviets have steadily increased their real defense spending, while ours has had a net decrease. In areas not limited to SALT I, they have launched ambitious programs to strengthen their strategic forces. At some future point, they could achieve a strategic advantage-unless we alter these trends.

"That is exactly what I want to do-with the support of the American people and the bipartisan support of Congress."

Rejecting "binding inlkage" of the arms treaty with other aspects of Soviet-American relations, Carter said the accord will not end U.S. "support for the independence of Third World nations" or his administration's "work for human rights."

"It is a delusion to believe that rejection of SALT would somehow induce the SOVIET UNION to exercise new restraint in troubled areas," the president said. "The actual effect might be precisely the opposite. The most intransigent and hostile elements of the Soviet power structure would be encouraged and strengthened by a rejection of SALT. The Soviets might well feel they have little to lose by creating new international tensions."

Carter added, in an argument he is likely to make repeatedly in the months ahead:

"A reliction of SALT II would have significane beyound the fate of a single treaty. It would mean a radical turning away from America's longterm policy of seeking world peace. We would no longer be indentified as a peace-loving nation."