Raymond Hunthausen has observed April 15 by witholding 50 percent of his income tax and publicly proclaiming it. He does not care for the way the Reagan administration would spend his money.

For several reasons, Hunthausen is not your run-of-the-mill tax rebel, of which there is a growing number in the country. He is the Roman Catholic archbishop of Seattle. He suggested to Christians last spring that they withhold 50 percent of their taxes to protest increased spending on the arms race.

None of his flock, who reside in the heart of the northwestern military-industrial complex, has publicly followed his example. But His Eminence practiced what he preached and, with his current Form 1040, enclosed a check for $125, half the amount due. Asked at a Seattle Press Club meeting if he was ready to go to jail for tax evasion, he said he was.

Hunthausen observed that the Internal Revenue Service has other ways of getting his money, perhaps by confiscating his savings account or garnisheeing his $9,000 salary, a sum that probably wouldn't cover a day's supply of paper clips at the Pentagon.

Washington state's economy is much tied to defense and nuclear enterprises. It has several nuclear power plants, and builds Boeing planes and Trident submarines. But since the archbishop made his startling suggestion of civil disobedience, his mail has been predominantly favorable.

Paul Weyrich, a spokesman for the right, calls Hunthausen "a radical of long standing." To the administration, of course, the archbishop represents a political threat that even extension of tuition tax credits for parochial schools and support on abortion do not begin to meet.

"I think the teachings of Jesus tell us to render to a nuclear-arms Caesar what Caesar deserves: tax resistance," Hunthausen told his congregation. He is one of many Catholic prelates who have taken a militant stand on the question. Remembering Vietnam, the White House had expected the hierarchy's solid support in the administration's anti-communist crusade. But on issues involving Central America and the "peace through strength" nuclear buildup, the bishops have proved a disappointment.

Hunthausen sent the other half of the $250 he owes the IRS to an escrow account for the World Peace Tax Fund, which is not in existence but will be if a bill sponsored by Sen. Mark O. Hatfield (R-Ore.) is passed. It would allow conscientious military tax objectors to designate the "military tax portion" of their returns for the Peace Fund.

But it is not law yet, and His Eminence could go to jail if the IRS decides to make an example of him. It poses something of a dilemma. No administration would want to send an archbishop to the slammer. But about 75 percent of the American people share his views about the arms race, and they could, if frustrated, start following his lead. Prosecution could be as dicey as nabbing the thousands who have failed to register for the draft.

Hunthausen supports the nuclear freeze, Ground Zero Week and the new "no-first-use" of nuclear weapons initiative. He is one of the interdenominational group of Seattle clergymen who have engaged in extensive dialogue with members of the Washington Legislature in the hope of persuading them to pass a freeze resolution.

Many Americans share the archbishop's views, although not his courage. The fear of the sound of the tax collector's step on the stair runs deep. The terror of the IRS audit stays their rebellion. Some take the coward's way of dodging taxes and give money to organized charities, some of whose blunt policies they find odious.

For instance, if you contribute to Amnesty International, you know it will turn in oppressive countries and help make the Reagan administration a little self-conscious about human rights. Closer to home, if you give to a scholarship, you counter in a small way Reagan's assault on college student loans. It's the wimp's tax revolt--and deductible, of course.

The income-tax forms are voluminous. But nowhere on them is a place where a person like Archbishop Hunthausen could specify what he did not want his money used for. There is no way, for instance, to tell the IRS you would rather see a food-stamp recipient have a vodka on you than to have your money go to a manufacturer of poison gas. There's no "preference" blank where you can write in: "Do not spend one dime finding a home for the MX--take care of human orphans."

Only one small box is set aside for choice. It asks you if you want $1 to go to a fund for the presidential election campaigns. It is not enough.

The archbishop is using his tax return as a weapon in the war against nuclear war. He is telling Ronald Reagan that until he listens to what the country is trying to tell him about nuclear morality he will get only half his allowance.