"When you accept the king's shilling, you sign aboard," the new White House chief of staff, Donald Regan, said the other day by way of explaining how an archconservative so hostile to the "media" as Pat Buchanan could be the new director of communications. That is, you loyally represent His Majesty's decisions. The royal image was arresting, coming from Regan, whose self-propelled switch from the job as Treasury secretary to the White House quickly stirred public speculation that he had sensed a role there for a "prime minister."

But it struck me all the more forcefully, having recently visited Europe. Variations on the same regal metaphor had come up with surprising frequency in conversations with Europeans trying to get a handle on the Reagan style of governing -- and in talks with American diplomats charged with helping them do it. In a part of the world long familiar with monarchies, parliaments, and a clear distinction between heads of state and heads of government, there is a natural inclination to relate the old world's ways with the dual capacities of the American presidency.

With Ronald Reagan, however, the normal curiosity gives way to amazement at the kingly quality of his rule. So, with the queen's ritual message to the British Parliament in mind, I listened to the president's State of the Union message to Congress. And there it was, just the way a high-ranking American diplomat in Europe had put it to me.

"Reagan speaks to the people in a quasi-regal way," was the explanation he offered. "He is free to hold up even the unattainable, as long as it is theoretically acceptable. 'Star Wars' addresses the essential insanity of nuclear war. A budget balanced by constitutional amendment says the same thing about the intolerability of endless deficits. Let the lsser people squabble over what the soothsayer just said. A president is supposed to inspire visions."

That is what Reagan dreamed of the other night before Congress: a balanced budget, "nuclear weapons banned from this earth forever," the whole bit. Most presidents are upbeat on this occasion. But Reagan has the magic to carry this annual exercise in inspiration to unprecedented heights.

What one French politician calls "Reagan's alliance with the people" is the envy of his European colleagues. But when the monarchical metaphor is put to Europeans, they also tend to share the judgment of American critics that the Reagan style is not necessarily to be confused with good government. As one of them put it: "A king has to have a prime minister."

Which brings us back to Don Regan. Whatever his view of his role, his quaint way of defining what it means to "sign aboard" the Reagan administration is a definition of exactly how things have not been working in the reign of Ronald Reagan. The president has a slate almost clean of solid foreign- policy accomplishment. Yet here lies the legacy he appears to care the most about: a safer, more peaceful, freer world by the end of his presidency. But it is there, as well, that you find the least evidence that acceptance of the "king's shilling" has ever come close to bringing everybody aboard.

Witness the prolonged inner conflict over arms control -- or the administration's recent presentations to the Senate Foreign Relations Committee's wholesale hearings on the fundamentals of U.S. foreign policy. Secretary of State George Shultz and Defense Secretary Caspar Weinberger seemed to be picturing two different worlds. In the interests of safeguarding every dollar of the defense budget, Weinberger had his reasons for talking about the Soviets as a growing military threat. Shultz, with an eye to the U.S. negotiating position, had his reasons for describing a shift in the "correlation of forces" frm the Soviet side to ours.

To the extent that the two saw things the same way, it had to do with the need to meet "Soviet encroachments" just about everywhere, but most urgently in Central America. Meantime, note was taken of the forthcoming arms control talks without any specific indication of a connection between doing business of mutual interest on arms control while doing battle, by proxy, on other fronts.

One is reminded of 1969, when the Nixon administration was setting up shop. Arms control talks were also on the front burner. But the Soviets were making trouble in Vietnam and elsewhere. Henry Kissinger was arguing, not for "artificial linkages" but for bringing the Soviets to understand "that they cannot expect to reap the benefits of cooperation in one area while seeking to take advantage of tension or confrontation elsewhere." He was saying that there was a need for a larger "conceptual" approach -- some way of giving context to conflicting bureaucratic interests and priorities.

That's what is missing in Ronald Reagan's king- size visions. The mechanisms exist for refereeing day-to-day disputes. What's lacking is the means for achieving what Kissinger called a "coherent approach or settled policy." And then, to carry it through, what's needed is somebody, if not the president, to play the part of prime minister.