The National Conference of Editorial Writers was taking a tour of the U.S. Navy Submarine Base here the other day when Secretary of the Navy John Lehman dropped in (by sheer chance) to present his pitch for something on the order of a 30 percent increase in the size of the Navy.

It's an interesting pitch, nicely encapsulating both the strengths and (to my mind) the potentially fatal weaknesses in the Reagan administration's national security approach--of which Lehman is perhaps the quintessential exponent.

In 1973 Lehman was a whiz kid on Henry Kissinger's National Security Council staff, happily working on a blueprint for a dream Navy of 700 ships. For seven years he stood by, helpless, while the Ford administration first cut the number back to 600 and Jimmy Carter, in 1977, cut it in half.

Now 38, he is no longer quite a kid. But the whiz is still there. He is bright, quick, witty, feisty and fiercely determined, with facts and figures at his fingertips. Standing on his klieg-lit quarterdeck, hands firmly on the podium, leaning into microphones, he does not exactly look like Horatio Hornblower. But he sounds like him.

"By 1989," he says, "I fully expect the Navy to be expanded to 600 ships (from today's 460) . . . a superior Navy manned by high-quality professional people, once again in command of the seas." And not just this or that sea--all seas. Lehman believes "there can be no such thing as a local naval war with the Soviet Union; if we engage the Soviet Union it is instantaneously a global naval war."

Parity won't do: "We must have unquestioned naval superiority" with the ability to "strike at enemy warships, ports, command and control, whatever controls his will to fight."

That is the heart of Lehman's naval doctrine: The Navy must be able to deal "simultaneously with conflicts in the Far East, the Near East, the Persian Gulf, the Indian Ocean, the oil lifelines around Africa to the United States and to Europe, the Mediterranean, the North Atlantic, the Caribbean and the Eastern Pacific."

Simultaneously? It's on that turn in the Lehman argument that I fall off. The spectacle of a global Trafalgar, conveniently confined to blue water and conventional weapons, strikes me as one of those "worst-case scenarios" that war-gamers play.

If Lehman is serious about striking at whatever destroys the enemy's will to fight, he is talking not just about ports and "command and control" but inevitably about air bases on the ground. He is no longer talking about "naval" warfare. And neither, in the view of a good many authorities, is he talking about a conflict that would remain conventionally non-nuclear for long or whose outcome would be decided at sea.

The question remains whether a Lehman-sized Navy is required to accomplish the primary mission of deterrence--of discouraging the Soviets from challenging vital American interests on the high seas. Here, of course, you get into much more difficult questions having to do with Soviet judgments and intents and the ability of the United States to define its vital interests and project its resolve to defend them with a full range of tactical and strategic military power.

The Lehman Doctrine, however, gives no more than lip service to the business of balancing risks in the allocation of scarce resources and the ranking of priorities: "The days of automatically following the trendy defense-cutting thoughts of some spokesmen in the news media, or Congress, or the administration are over." Citing treaty commitments to 40 nations, he bluntly challenges Congress either to provide the necessary naval forces "or simultaneously decide which of our major commmitments we're going to drop, becaus we can't meet them with the current Navy."

Here, Lehman has a point. The Navy plainly couldn't fulfill all the missions assigned it today--and certainly not simultaneously. Congress has skimped on unglamorous spare parts and ammunition. Ships are undermanned. The Navy is short some 20,000 petty officers who take years to train.

While recruitment and retention have both taken a sharp turn for the better, this owes as much to recession as to sweetened pay and allowances. Lehman will have to find another 100,000 men for his 600-ship Navy, and even he admits "personnel will pace everything."

If John Lehman's Navy is hostage to the availability of qualified manpower, it relies, as well, on cooperation from the Soviets. It would require extraordinary forbearance for the Soviets to stand idly by while the United States, having proclaimed its own naval inferiority, sets out to achieve unquestioned "superiority."

Lehman is an eloquent advocate of the best of all possible worlds. The question is how well his dream Navy fits into the real world.