The Pentagon and some of it's usually staunch allies squared off yesterday in a fight over whether the Army should be cut by 25,000 to make its quality higher.

Sen. Sam Nunn (D-Ga.), who is championing the reduction in force, called a press conference on the eve of a Senate vote on the issue to charge unnamed civilian leaders at the Pentagon with waging "a campaign of deception" to cover up the sorry state of the Army.

Nunn said the Army tried to cover up test results showing failure rates of up to 91 percent in performance of military specialties. The Army shot back with a statement that the tests cited were "outdated."

With Chairman John Stennis (D-Miss.) of the Armed Services Committee one of four senators flanking him during the press conference, Nunn said "the question I have is, 'Who will save the Army?"'

Congress could help save the Army, Nunn contended by adopting his proposal to reduce it to 750,300 people by the end of fiscal 1981, a cut of 25,000. Then he argued, recruiters could concentrate on quality rather than quantity in filling the ranks.

If the Army started signing up a higher percentage of high school graduates that it has in recent years, it could earn back the 25,000 cut. Starting at a base of 52 percent, every time the proportion of male graduates went up by 1 percent, the authorized total strength of the Army would go up by 1,250 people. Reaching the level of 72 percent graduates would push the total back up to 775,300.

Pentagon leaders violently oppose the plan, asserting now is not the time to cut the Army by two divisions.

Sens. Carl Levin (D-Mich.) and Ernest Hollings (D-S.C.) will take the Pentagon's side in today's likely Senate fight over the Nunn amendment.

"I believe that reducing our Army end strength is exactly the wrong signal to be sending to our allies and to the Soviets in this period of increased international tension," Levin said. He added that Gen. E. C. Meyer, Army chief of staff, had warned that the cut would aggravate, rather than relieve, personnel problems.

Pentagon officials also are dropping broad hints in lobbying against the cut that some training bases, like New Jersey's Fort Dix, may have to be closed if the size of the Army is reduced.

Stennis, in backing Nunn at the press conference, said "the only way to avoid the Selective Service System is to improve the quality" of the Army.

To the distress of Nunn and other Army critics in Congress, Secretary Clifford L. Alexander has insisted that the Army is in good shape.

Without naming Alexander, Nunn said yesterday that Army leaders "appear to want to keep the facts about the serious deterioration of quality cloaked from public scrutiny and to avoid congressional actions to insist that quality be maintained in the Army."

Nunn read off a list of performance test results showing failure rates of 91 percent among merchants, 83 percent among transportation specialists and 69 percent among communications people.

"We must not permit the U.S. Army to become a Job Corps equipped with tanks and nuclear missiles," Nunn said, "yet that is the path we are moving towards."

Alexander issued a rebuttal last night, declaring it "patent nonsense" to accuse the Army of covering up its manpower problems. "Most of the statistics cited by Sen. Nunn are shopworn and outdated."

The Army secretary said skill tests demonstrate "beyond a shadow of a doubt that more and more soldiers each year are better able to perform their military missions.

"To cut the Army's end strength by 25,000 as Sen. Nunn recommends," Alexander said, "is certainly not a signal we need to send to our adversaries."