Defense Secretary Caspar W. Weinberger today renewed his counterattack against those calling for smaller increases in defense spending, telling an American Legion convention here he is "concerned about some of the economic arguments which are being advanced against needed strengthening of our defenses."

Though Weinberger did not identify who is advancing those arguments, he told the Legionnaires that "as a former director of the White House Office of Management and Budget, I am particularly aware of the need to avoid excess or unnecessary spending."

The current OMB director is David A. Stockman who, at the moment, is a principal force in the administration attempting to convince President Reagan that a significant scaling down in the rate of military spending increases is needed to balance the federal budget by 1984.

On his return to the White House, Reagan said, "No department is going to be exempt, but we are not retreating from our program of building up our defenses." United Press International quoted one White House official as saying of Weinberger, "He won't give up, will he?"

Weinberger told the Legion group that, while it is necessary to avoid excess, "it is vital also to keep in mind that as important as economy and efficiency are -- and I think they are indispensable -- our assignment is nothing more or less than to safeguard the United States" by acquiring enough strength so that no nation dare attack.

"We are far more likely to maintain the peace if we are strong enough to deter war. There is no way to wish aside the realities in the world which demand that we rearm and do it expeditiously," the secretary said in his prepared remarks.

"Fortunately, the only sacrifice we must make today is to be prepared to be strong enough to deter aggression. That is the most important task this administration was elected to carry out," Weinberger said, in a line that essentially defines the dilemma of the Reagan administration as it seeks to balance a commitment to huge defense increases with a commitment to cut taxes and balance the budget.

The debate now going on in the administration, however, is about how much to slow or stretch out the rate of increase and not about cutting defense back to levels lower than previous years. White House officials have said the president is prepared to trim as much as $30 billion in the 1983-84 defense plan. The total Reagan military budget for the next five years, however, stands at $1.6 trillion.

Weinberger has made clear during this trip that he has not had his day in court with the president on this issue and that he will argue against any major revisions.

In the meantime, Weinberger says, the Pentagon is reassessing its budgetary priorities so that if the president does call for a reduction, the armed forces will have various plans ready.

Earlier in the day, the secretary, who served as an infantry officer in the Pacific during World War II, had breakfast with marines in nearby Camp Smith and then watched small-scale maneuvers by units of the Army's 25th Infantry Division at Schofield Barracks.

Weinberger seemed to enjoy slogging around the muddy training ground, his white shirt and presidential cufflinks protruding from an infantry shirt. He was allowed to "shoot" one soldier in a demonstration of a new laser target system. The defense chief, ever the gentleman, apologized afterward to the GI for shooting him.

Weinberger also scored a direct hit, at least according to the automatic laser system, on the first shot at a tank in a simulated firing of the Dragon missile. After his breakfast meeting, Weinberger told the gathering of enlisted people that he wanted to talk a little about "our commander-in-chief" because "I don't know how many of you have had a chance to meet or talk with him."

Of President Reagan, Weinberger said, "He is, I suppose, one of the most underestimated men in the world. People consistently do not realize or recognize the qualities he has, or appeal he has, or realize the reasons for that appeal. He believes completely and fully in what he is doing, has developed a philosophy and follows it consistently."

So people trust Reagan, Weinberger said, and compared the admiration for his boss to that once given President Eisenhower.

Weinberger's prepared remarks here were laced with sharp attacks on the Soviet Union, from whose "secret depth arises a threat to our liberty unlike any we have ever seen," he said. The Pentagon chief talked of Soviet "changes in leadership that result from internecine power struggles held behind tightly sealed doors with men's very lives often at stake."

Weinberger talked of "that grim land with its economic problems and shortages of grain" that, since 1973, has increasingly used diplomacy, military aid, proxies, support of terrorism and invasion to make gains overseas.

"The Russian defense buildup that now threatens this country," he told the Legionnaires, "was done during, and despite, the period called detente and the attempt to limit arms by treaty." Those steps restrained only us, not the Soviets, he asserted.

Weinberger said Vietnam inflicted a heavy blow on American youth because it asked them to fight a war "we did not intend to win. I can tell you," he added, "with absolute certainty that President Reagan will never make such an unpardonable demand on the youth of America."

Weinberger said the growth of Moscow's capabilities required a global strategy that could no longer be adequately dealt with or described by such "old slogans" as preparing to fight 1 1/2 or 2 1/2 wars at one time.

"We cannot shape our forces and doctrines as we have in the past solely on the assumption that there would be an initial conventional defense with a quick nuclear escalation and thus an early end to the war.

"Our loss of nuclear superiority makes that unthinkable," Weinberger said.

"So I have instituted the strategic changes necessary to improving our ability to fight a long war, as we may well have to do if we value our freedom.

"These changes mean that we will have to pay more attention to our industrial base, to improving mobilization planning, to mobility for our forces, to air and sea lift and to powerful naval forces," he concluded.