If the Defense Department had to slash $60 billion out of the fiscal 1987 defense budget to meet the belt-tightening requirements of the Gramm-Rudman-Hollings deficit-reduction law, how could it be done?

Budget experts estimated that if the automatic, across-the-board cuts required by the law were to take place next fall, the Pentagon would have to give up at least $60 billion to shrink the federal deficit to the $144 billion required by Gramm-Rudman-Hollings.

The law does not allow programs to be canceled to reduce spending, nor could the Defense Department spare one account by cutting deeper into another. Consequently, Defense Secretary Caspar W. Weinberger would have to cut about 20 percent from each of the 3,250 line-item accounts in defense to reduce budget authority by $60 billion.

How big is $60 billion in terms of Pentagon programs? Very big, even for the Pentagon. Here are the programs that can be bought for $60 billion in fiscal 1987:

The Strategic Defense Initiative, the Army Apache attack helicopter, the Patriot antiaircraft missile, the M1 tank, the Navy F/A18 fighter, the Trident 2 submarine and its new missiles, the Aegis cruiser, the Arleigh Burke destroyer, the Los Angeles attack submarine, the Air Force C5 transport, the F15 fighter, the F16 fighter, the MX missile, the Midgetman missile, and the military's communications and intelligence equipment, which alone will cost $30 billion in 1987.

President Reagan is trying to avoid radical surgery by presenting Congress with a budget that reduces the deficit to the required level by drastically cutting many nondefense programs, even as the defense budget would grow 8 percent above inflation to $320.3 billion.

Virtually no one on Capitol Hill believes such an increase is plausible. Lawmakers have spent much of the past two days pronouncing the Reagan budget dead. But the prospect of a Pentagon emasculated by $60 billion in automatic cuts is considered so unnerving and absurd that the very vision is used by Republicans and Democrats as proof that some sort of compromise -- through a revenue increase, or cuts, or both -- will have to emerge in coming months.

"They'll sober up in about a month to what they have done to defense with Gramm-Rudman," Rep. Thomas J. Downey (D-N.Y.), a member of the Ways and Means Committee, said of his fellow lawmakers. "They're still on a bit of a champagne high from getting Gramm-Rudman through last year . But I see revenue measures coming by September."

Many Defense Department accounts would have to shrink by $8 in budget authority to save $1 in spending. That is because the money authorized by Congress for a research project, for example, is spent incrementally as work progresses.

Pay for soldiers and sailors is different. Almost all money placed in the budget account for salaries is spent the same year it is appropriated. This means Congress or Reagan could make immediate reductions in spending by kicking people off the Pentagon payroll.

However, the House Armed Services Committee estimated that 420,000 soldiers, sailors, airmen and Marines -- or roughly one-fifth of the entire armed forces -- would have to be discharged to reduce military spending by $30 billion in the coming fiscal year.

Congress could forestall the automatic deficit reduction measures by passing a budget that lowers the fiscal 1987 deficit to the $144 billion level, but that likely would require deep cuts in defense.

"That's a choice between suicide and murder," Rep. Les Aspin, (D-Wis.), chairman of the House Armed Services Committee, said.

Aspin and others predict that the politicians will let the mechanical hand of Gramm-Rudman-Hollings commit murder for them.

However, Downey is among a growing number of lawmakers who believe that the combination of falling oil prices and the Draconian actions required to carry out Gramm-Rudman-Hollings will forge a congressional-administration alliance to raise more money for defense and domestic programs.