Washington is not sure what to make of the power-swap in the president's inner circle. Everybody is expected to have an opinion about why Treasury Secretary Donald T. Regan is going to be the president's chief of staff and chief of staff James A. Baker III is going to be treasury secretary. Even the town's most dedicated Baker-bashers, the hard-core conservatives, cannot be sure what it means.

Washington has not learned that the the question to ask of this administration is not "why" but "why not."

It is considered a cop-out to buy the obvious explanation -- that Baker and Regan had "had it up to their keisters" with their present chores and that Reagan agreed each man would benefit from a change of scene.

The Democrats were a bit distracted when the news hit. They were busy absorbing what they hope will be the last awful blow from their recent attempt to dislodge the president, the indictment and guilty plea of the husband of their erstwhile vice-presidential candidate. But they managed to cluck about the way the job transfer was accomplished. It illustrated once again, they said, that Ronald Reagan is a bystander president.

Nobody even bothers to suggest that the switch was the president's idea. It is freely acknowledged that Regan and Baker began cooking up the scheme some three weeks ago. They had it brought to the Oval Office for ratification. The Gipper agreed.

You see, the Democrats cackled, "He is not in charge."

They still do not understand that the president doesn't mind if he looks more like a figurehead than an activist.

Imagine how it would have been in Lyndon B. Johnson's time if it had gotten out that subordinates had exchanged empires in secret and gone to him with the papers to sign. He would have thought summary firing too good for them.

In Jimmy Carter's case, a scenario doubtless would have been dreamed up to show that the president, during a walk in the woods, suddenly had been struck with the symmetry and equity of an even swap and put it in train.

But that is not this no-sweat president's way. He does not regard it as his job to mediate the public quarrels in his Cabinet even on arms control. His team went to Geneva squabbling.

He doesn't have regular news conferences where he can be asked whose side he is on. The people around him, including his wife, say -- and editorial writers write -- that he should meet the news media at regularly scheduled, announced intervals. But he had one in July and waited 5 1/2 months to have another.

The public doesn't get exercised about the fact that he doesn't have news conferences, so why should he? And the country, which, paradoxically, admires his "strong leadership," doesn't care that as captain of the ship of state, he frequently goes below while the crew handles the winds and the waves.

He has this liberating assurance on good authority. The Washington Post and ABC News, in April 1983, conducted a poll in which people were asked, "Who do you think makes the important decisions, Reagan or the people around him?" Fifty-seven percent said it was the advisers. As to whether this was the way it ought to be, 60 percent said yes.

Ironically, the Democrats and the conservatives are united in thinking that the transfer of Baker from Reagan's right hand is a good thing. Both groups feel he may do them less harm in Treasury.

The Democrats note that Baker did the president no end of good, from running the country to seeming to age in office in his stead. His dispassionate, cautious manner, his command of legislative detail, made him an incomparable congressional negotiator. His consummate professionalism is not expected from Regan, who is given to confrontation and cheerleading.

The night of the announcement, the right-wingers were gathered at a memorial dinner for Larry McDonald (D-Ga.), the congressman/Bircher who died in the KAL 007 disaster. They applauded the removal of "the prime minister," as they call Baker, because "his eyes glazed over when we tried to talk to him about our issues." But his successor is not 100 percent. "When Regan ran Merrill Lynch, he gave PAC money to Democrats," it is said.

The Democrats think that without Baker to bail him out, Reagan will flounder. The conservatives think that without Baker to steer him wrong, Reagan will again be himself. But both groups know Baker will remain still just a phone call away from the Oval Office.