TWENTY YEARS AGO a new president of the United States took the oath of office. if he were alive now, he would be 63. He would still be younger than his successor who took the oath 18 years ago. Ronald Reagan on Jan 20, 1961, had not even begun his political career, although he was six years older than the John F. Kennedy who was inaugurated.

One observer at the time looked forward to "a raree show worth the penny." It has been impossible in the past two weeks, with all the talk of a new elegance and the expectations which people now have, not to look back to that inauguration. The mere thought makes one very comfortable. The greatest nation in the world has in 20 years slid from luster to glitter.

Those of us who have been and still are severely critical of the political method of John Kennedy can nevertheless feel a twinge of pain. It should not in two decades have come to this. We ought not to be trying to applaud an inauguration week which was no more than a tawdry display of opulence.We should not be reading greatness into an inaugural address which has no chance of making an anthology of political oratory. Hollywood in a mere 20 years has come to Washington; we ought not to brush aside the degeneration. The coinage of political life has been debashed.

Two decades may not be long under the gaze of eternity. But they are long in the nistory of a nation. What is it which even we who were and are critics of Kennedy can say has been lost in that time? There have been five inaugurations since Jan 20, 1961. None of them has given people any reason to be inspired to public service as a noble and selfless pursuit.

Lyndon Johnson was able to seize a mood which he himself had not created. He did it well in the flurry of his first two years, before and after his inauguration, and in many of his programs there was an inspiration to serve. It must not be forgotten that the idealism which Kennedy had excited in the young was carried into much which Johnson did until the war in Vietnam broke their hearts.

But what beacon since then have the young been given? Gordon Liddy is a powerful attraction on today's campuses. His meretricious talk of the power of "will" is all the students have in place of genuine purpose. They are titillated by the voguish appeal of libertarianism, which is only selfishness exalted into a moral system.

Who cannot look back to 1961 and wonder, in the depths of his heart, what has happened in the span of a generation? Richard Nixon, Gerald Ford, Jimmy Carter, Ronald Reagan: How long can America afford this declension of its public spirit? How long can it survive it? No bright pennants now capture the ardor of a people as they encounter their destiny as the champions of men's freedoms and highest hopes.

Ribbons are not a substitute for banners.

There was much that was false, some that was even laughable, in the inauguration 20 years ago. Twelve dozen writers and artists, composers and philosophers, scientists and cultural figureheads tumbled into Washington in ecstasy. The heady atmosphere was caught by Lincoln Kirstein: "Mrs. Kennedy had said that she was going to redecorate the White House, and every American abstract expressionist had had palpitations."

It is possible to be still critical of "the politics of expectation" and yet grant that it was at least better than what we have had since.Four presidents in succession have now come to power promising a reduction of expectations. It is both sad and alarming that the reason most often given for welcoming the new president is that he is amiable. Whenever before has a great nation set forth amiability as a virtue in those called to lead it? Amiability is in this context a euphemism for mediocrity. It is mediocrity we now applaud.

Eight years ago on the eve of St. Valentine, Richard Nixon gave a copy of my book, "The Kennedy Promise," to a visitor who called on him at the White House. That visitor was Ronald Reagan. I was grateful for the 15 percent royalty on the single copy -- did it come from the public treasury, or had he gone personally to a discount store? -- but I was flabbergasted at such a donor and recipient. All I can say now is what I said at the time; that both would take the wrong lesson from it.

It is 12 years of steady lowering of expectations which make even a critic of Kennedy like myself look back to that other inauguration with an ache. It is not even against President Reagan himself that the real burden of my criticisms today are leveled. Jimmy Carter came to power proclaiming a reduction of expectations to be his goal as the leader of the Democratic Party.

The truth is that the politics of America has since 1965 been dispirited. One simply does not believe that Reagan will place it again in command of a great alliance in the world when he inspires its people to no high expectations even of themselves at home. You do not stir a nation to greatness abroad by snatching the food stamps from the poor at its checkout counters.

The president may proclaim a foreign policy of strength and firmness; his director of OMB wishes to run it by saving candle ends. If anything may be said from history, it is that a government which is not active at home will not have or excite the will to be active abroad. The conflict which lies at the heart of the new administration is between a policy of domestic docility and the promise of assertiveness overseas. The two do not go together.

This used to be the true understanding of the liberals: Wilson and Roosevelt, Truman and Kennedy, even of Johnson, from whose tragedy the wrong lessons can be drawn. It was the understanding of the greatest Republican president of this century, Theodore Roosevelt, as it was also the understanding of the greatest Tory prime minister of this century, Winston Churchill. Republican and Tory though they were, however, no one would label them as conservatives. As they were active abroad, so they were at home. This is a correlation whch conservatives can never bring themselves to face.

The canker which is at the core of this administration is the canker which has caused America's difficulties for 12 years. It is impossible to assert a new nationalism in foreign policy without asserting the same nationalism in domestic policy. If the national goverment is weakened here, it will be that much be weakened overseas. It is the sheeret deception to base a strong foreign policy on a weak federal government.

One cannot spend most of one's inaugural address telling the people that government will inspire them only to look after their pocketbooks, and then tack on to the end an appeal to a warrior of several wars ago who was ready to sacrifice all in the service of his country. If you wish people to be ready to die, you must first excite them to live.You do not make your country soldierly simply by sewing the ribbons of an unscarred general to the tunic of its State Department.

This administration is at heart passive. It is for this reason that one fears that it will seek to prove itself by rash adventure. Every prescription is there for America to go to war in the wrong place, with the wrong ambitions, for the wrong objectives, with the wrong weapons, at the wrong time, and more probably than not suffer yet one more defeat for its pains.

What we will soon discover is that a national government cannot be created by direct mailing. What at least was present 20 years ago was the assertion that without a government there is no nation. This truth may have been tricked out in false colors. So be it. But government was held up as a proper object of our allegiance and even our affections.

That sense has been lost in America. Like its servants who are hounded into the pillory, government has itself been all but stripped of honor. It would be much harder than 20 years ago to find young people now who believe that it is more honorable to serve government than to serve Mobil Oil. The emotions which attended the release of the hostages were cheap in the sense that they were a patriotism that costs nothing. The public in America is dissolved. Who then will the president lead to battle?