It is customary in the second January after each inaugural ceremony to write a midterm assessment of a presidency. That is what I set out to do. But it quickly became clear that in the case of Ronald Reagan, something else is required.

What we are witnessing this January is not the midpoint in the Reagan presidency, but its phase-out. "Reaganism," it is becoming increasingly clear, was a one- year phenomenon, lasting from his nomination in the summer of 1980 to the passage of his first budget and tax bills in the summer of 1981. What has been occurring ever since is an accelerating retreat from Reaganism, a process in which he is more spectator than leader.

One measure of that transition was last week's Gallup Poll showing Reagan trailing two leading Democrats in trial heats for the 1984 election. Former vice president Walter Mondale had a 52-40 percent lead, and Sen. John Glenn of Ohio had a 54-39 advantage.

Such leads for opposition candidates are extremely rare at this stage of the cycle when all presidents, including Reagan, enjoy an aura of authority. But presidential polls change. Much more significant is the way in which power is moving away from Reagan in the ongoing work of government. What began as a process of delegation is rapidly approaching abdication.

Look at the world scene. The Middle East peace effort is at a crucial juncture, so special envoy Philip Habib is hard at work on the problem. The Far East demands attention, so Secretary of State George Shultz puts in a long Saturday of briefings, in preparation for a trip to China, Japan and Korea. Western Europe stirs in response to a peace initiative from the new Soviet leader, so Vice President George Bush schedules a round of high- level talks in the European capitals.

Meanwhile, the president--back five days from his most recent California holiday--is photographed in sports clothes, heading off for a weekend at Camp David.

To be sure, there is important, unfinished business to occupy him. As he left for Camp David, final decisions had not yet been made on the budget he submits at the end of the month. But less and less effort is made to pretend that Ronald Reagan is managing those decisions on a day- by-day basis. On the contrary, his own aides leaked word that he had skipped last Friday's budget session, and one senior official told The Post's Lou Cannon and David Hoffman that Reagan "is probably the most detached president that has served in that office in a long, long time." 2 His detachment is extraordinary, in face of record unemployment and a fiscal crisis that threatens intolerable deficits of $200 billion or more each year for the foreseeable future. Republican congressional leaders, administration economic and budget officials, the senior White House staff and the inner-circle Cabinet members were struggling all last week to find a way to escape the mess that threatens the nation's and the world's long-term economic prospects.

They brought their ideas to Reagan, who sent them back to work again. Why? Not because he was raising important questions the others had failed to consider, or suggesting alternatives they were not imaginative enough to see. No one pretends that Reagan contributes to the policy-analysis process in that way. His role was to ask how the measures they were recommending could be reconciled with his promises of 1980-81 and the simplistic rhetoric of his 30 years on the conservative banquet circuit.

The job, as Cannon and Hoffman described it, was "to make Reagan recognize that his most cherished goals could not be reconciled"--not with each other and not with external realities.

The real work of governing at this point is to deal with the complexities of the world scene and to remedy the errors and excesses of domestic policy that marked the year of Reagan's ascendancy--to slow the runaway growth of defense spending, to recapture some of the squandered revenue base, to cancel the foolish indexation of tax rates before it goes into effect.

In that process of mid-course correction, Reagan is less the man out front than the barrier to be overcome. Even if he is persuaded to lend his voice to the effort, he will be the tag-along.

At some point down the road, the phaseout of the Reagan presidency will confront the Republican Party with an awkward but vital choice of its future leadership. At that point, those who are now cooperating in easing the transition from Reaganism--the Bushes, Laxalts, Bakers and Doles, plus the key members of the White House staff and Cabinet--may choose up sides in the struggle for succession. Some may see a signal of that moment's imminence in Sen. Howard Baker's hint that he will leave the Senate to seek the presidential nomination when Reagan steps down. But the country is fortunate that, for now, these men are putting aside their personal ambitions and working together to fill the vacuum of leadership that Reagan's phaseout has left.