Sen. Mark Hatfield is leading a charge to reduce defense spending. Ordinarily, this would give the administration as much pause as hearing that the Salvation Army was mounting an attack on the White House.

But one of the few disadvantages, from Ronald Reagan's point of view, of having a Republican Senate is that Hatfield, the most notorious dove in the president's party, has become chairman of the Senate Appropriations Committee, where he is in an excellent position to do something to the Pentagon's enormous allowance.

Hatfield, an organizer of congressional prayer breakfasts, is genuinely religious--his colleague and former student, Bob Packwood, calls him "a Christian gentleman." His philosophy on foreign policy was formed in his own experience.

A Navy officer in World War II, he was among the first Americans to enter Hanoi after the Japanese left and was persuaded that Ho Chi Minh was a nationalist leader who could be a friend. Also, he found himself among the ruins in Hiroshima--he knew the bomb had saved his life but wondered about the cost to mankind--and was forever lost to the Reagan enthusiasm for nuclear weaponry.

During the debate on the AWACS, which he "wearily" opposed, he spoke of the "tragic flaw" in our foreign policy: "The notion . . . that we as a superpower, can somehow enhance the prospects for an enduring peace by arming nations and cultures with ever more sophisticated weapons of war. We are not the only nation of the world that embraces this notion, but because of our status as the world's largest peddler of arms, we have a special moral requirement to examine it."

The president noted this strain in Hatfield. When asked in an interview with Middle Eastern specialist Trude Feldman about Hatfield's negative vote, he replied airily, " . . . he is opposed to any arms sales. He believes the world would be better off if there were no arms."

Almost any other senator, particularly a member of the newly martial GOP, would be horrified by the suggestion that he was a closet unilateral disarmer. But Hatfield, when asked for his reaction, seemed pleased that the president had noticed.

How much he is heeding Hatfield is entirely another matter. The president has more or less committed us to an open-ended arms race in the Middle East, plans to spend $181 billion in arms and has cut a mere $2 billion from the Pentagon budget.

The dispute within the administration over the need to do more ended in a triumph for Secretary of Defense Caspar Weinberger. Budget Director David Stockman, when asked if anything would be done to at least slash waste and fraud in defense, replied in effect, that that was done and over.

Hatfield thinks that an additional minimum $2 billion can be sliced out of the defense budget by lopping off $45 million worth of public relations specialists, not to mention a $2.3 million helicopter that can't fly, a $195 million radar system that doesn't work and 480 historians and museum curators.

He also is taking aim at the MX, and the B1 bomber.

He talks about these things in his office at a turn-of-the-century open-front desk with an old-fashioned telephone on it.

"We are accelerating the erosion of the infrastructure of our society. We are weakening our education system--they want to cut the Fulbright program in half; we are endangering health care, transportation, resource management. What we are saying is that we can let it deteriorate because we are building more bombs. But if we don't have a strong economy and a strong people, what is our national security?"

This is, of course, precisely the same kind of "priorities" argument that he made, to little avail, throughout the Vietnam war.

The only reason anyone might pay attention in this era of increased, and apparently popular, belligerency is that as chairman of the Appropriations Committee, he has a fighting chance of making it stick.

Also, he has allies within his own party, members who for reasons often different from his are digging in their heels against the additional domestic cuts demanded by the president in the interests of Pentagon spending.

Hatfield is in charge now, which means that he can tell those who wish to add billions for defense projects that they must subtract those sums from favorite domestic projects.

For instance, Sen. John Stennis, ranking member of the Armed Services Committee, is a doughty ally of the Pentagon. But he is also interested in the Tennessee-Tombigbee dam. Hatfield will inform him that he can't have both.

It is doubtful that the mild and thoughtful senator from Oregon will ever convince the hawks that he is right in thinking that arms do not produce peace. But after 15 years of crying in the wilderness, he has a gavel in his hand, which is the only weapon he wants.