This is a strangely calm moment in Washington. A president is to be sworn in today, but because he is a second-term president (our first in quite some time) and, more important, because he is Ronald Reagan, there is relatively little sense of drama and excitement to the occasion. We say this not to put it or him down but by way of noting an unmistakable air of familiarity and naturalness to the proceedings -- and to the man.

We, all of us, know Ronald Reagan. We know him as an authentic individual personality at peace with his own strengths and limitations, as a man with remarkably little distracting pride, bias or bile, and as one who in his first four years delivered, or at least tried to deliver, what he had promised.

These qualities have not stifled all political debate in the land -- far from it -- but they have produced a pervasive mood of acceptance of his power. His supporters celebrate it, while most of those who did not support him in November grant the validity of his victory as a victory and not a fluke. This is in its way his biggest triumph of all.

It is worth recalling the tone in this city four years ago today. The president leaving the White House had won respect for some of his personal qualities and accomplishments (history will treat him better in the years to come), but the consensus judgment was that he had failed to use the powers of the office decisively and consistently and it had cost the country dear.

Who can forget the bittersweet resolution of the hostage crisis during the very moments that Mr. Reagan was taking his oath? He came down from the stand and it was a new situation: the country was hostage no more.

The more serious and creditable thing, however, is that Mr. Reagan has largely sustained this spirit. Some of it, to be sure, he has done with mirrors. Five Americans are hostage today in Lebanon, where he conspicuously invested and then insouciantly disinvested American prestige; neither for the five nor for the larger policy failure has he been taxed severely. Still, calls for a display of "will and moral courage," in his words, have faded, chiefly because in the judgment of most people the president has displayed them. He was lucky, but he made a good part of his luck.

Four years ago, too, there was a widespread sense that Jimmy Carter had let a great sickness -- inflation -- seep through the country. Inflation was described by a dedicated Reaganite as "the transcendent issue of our times." It seemed the result not merely of certain policy choices and international conditions but, again, of a shortfall of presidential resolve.

Mr. Reagan's economic policies remain, deservedly, under sharp attack in many quarters. Yet it is undeniable that inflation has been mercifully trimmed and that the substantial economic and social strains that still bedevil the country are more easily handled within the traditional political context.

Oddly the president has abdicated the usual presidential responsibility to stand at the economic helm. Given his genial bent for discredited economic theory, however, there is a certain cheer that he is taking a holiday. This is one of the astonishments of the Reagan presidency: Leadership is there, or seems to be there, even when it's not hooked up to policy.

Mr. Reagan entered the White House four years ago very much identified with one end of the political spectrum. Politically, his achievement has been to hold most of his original base, as disgruntled as parts of it may be, while gaining the acceptance or at least the tolerance of other quarters.

That he won bigger in 1984 than in 1980 is the most substantial tribute our system pays to this kind of performance. It suggests that, as bitterly as some have opposed him, he has succeeded in the large purpose of becoming president of more of the people. This he has done with an office that, before he held it, was said to be a shrinking cage for its successive inhabitants. Mr. Reagan has renewed the possibilities of the presidency. He has a rich opportunity to serve, in his fashion, the great American goals of justice and peace.