During the past several years, the Senate permanent subcommittee on investigations has probed labor racketeers, offshore banks and waterfront corruption.

This year the subcommittee and its parent committee have taken on the Pentagon and the way it develops and buys ships and weapons.

Their shift of attention indicates a trend on the Hill.

As the Reagan administration's weapons procurement program has fattened--from $64 billion in fiscal 1982 to a projected $123 billion in fiscal 1985--it has presented an increasingly inviting target. Though sometimes for different reasons, both liberals and conservatives have intensified their scrutiny of Pentagon procurement practices.

"Critics of the defense budget can no longer get away with arguing that we need less defense, or that the Soviet Union poses no threat to peace," Paul Thayer, deputy secretary of defense, said recently. "Now we are hearing a new argument which is more popular and more seductive . . . , the argument that we can get more for less."

But recent suggestions that the Pentagon may be getting less for more have come from such true believers in the Soviet threat as Sens. William V. Roth (R-Del.) and John G. Tower (R-Tex.), from a panel of top business executives appointed by President Reagan to study government in action and from Thayer.

* In the same speech in which he attacked the "more for less" philosophy, Thayer also charged that shoddy work by arms manufacturers adds 10 to 30 percent to the cost of weapons and equipment.

* The General Accounting Office, the investigating arm of Congress, reported that government contracts had allowed several shipbuilders to earn profits that many businesses might envy, as high as 61 percent in one case. The report confirmed allegations that Adm. Hyman G. Rickover had leveled before he retired.

* Tower, chairman of the Senate Armed Services Committee and normally a friend of the Pentagon, announced a series of hearings beginning July 26 to investigate how well the Pentagon is managed, including the procurement and testing of weapons. Among other issues, Tower said, the hearings will focus on the possible negative effects of "multiple levels of review and longtime lapses" in weapon development.

* The President's Private Sector Survey on Cost Control reported that the Pentagon could shave at least $28 billion a year from its budget through better management, with the largest chunk coming in weapons procurement. While acknowledging the difficulty of managing the department, the commission criticized almost every aspect of weapons testing and procurement.

The council examined 25 major weapons systems under development and found that their costs rose from an estimated total of $104.8 billion seven years ago to $339.2 billion today. Its report stressed that procurement is growing faster than any other component of the budget.

* The GAO also charged that billions of dollars worth of weapons are purchased before the Pentagon knows whether they will work because it does not test them adequately.

The department's deputy inspector general confirmed the charge through separate audits, saying top officials often push for full production before test results are in and sometimes do not learn from lower-level officials or contractor personnel the significance of test failures.

By last week, Pentagon officials were more than annoyed by the chorus of criticism. Thayer came downstairs to the Pentagon briefing room to attack the "misinformation" being put out.

"The U.S. is producing the best equipment in the world," Thayer said. "The testing procedures are much better than they are in any other part of the free world, and from what I know, any phase of the Russian testing complex with the possible exception of ICBMs," or intercontinental ballistic missiles.

Reagan administration officials acknowledge that the testing program, like the procurement system, has not worked well in every instance. But they have instituted several improvements, they say, and should not be criticized for past sins.

"It doesn't make any difference what you look into," Roth responded. "They come back and say, 'Everything's fine.' "

Roth said his home-state surveys show a consistently declining level of support among voters for high defense budgets.

That perceived decline so far has not translated into many lost votes in Congress for the Reagan arms buildup. But many congressmen are looking for ways to prove to the voters they are taking a stand against unwarranted military expenses, according to Rep. James A. Courter (R-N.J.), a leader of the informal House Military Reform Caucus.

Critics of the Pentagon once again were making their case by pointing to the Maverick missile, an anti-tank weapon whose troubled history was further revealed by a memorandum released last week. The "imaging infrared" Maverick is a "smart" weapon that is supposed to be shot from a fighter jet and then find its target by locking onto the source of heat.

But tests consistently have raised questions about how well the Maverick would work in combat, and the Pentagon's chief of testing acknowledged recently that he recommended against production increases in the $3 billion program.

Last week the Pentagon released a memo showing that top officials had decided to increase production despite that recommendation and despite test results they considered "inconclusive."

Thayer said the Pentagon is confident the problems can be resolved. And, advancing an argument that often has persuaded even critical congressmen, Thayer added: "The Maverick has had its problems, but right now it's needed."