It takes extraordinary good luck to become President of the United States, but once having been elected, a President must sustain extraordinary bad luck to not be re-elected. Since 1912 only one elected President (Herbert Hoover) has been defeated for a second term, and it took the worst depression in the nation's history to do it.

This singular habit of the American electorate seems to have been overlooked or heavily discounted by those critics of Jimmy Carter who already are writing him off as one-term President. If precedent holds, it will take far worse years than 1977 to bring the incumbent down in 1980.

While Carter certainly proved himself an able campaigner for the presidency, and, consistently helped him along, and assessing the prospects for 1978, it seems to be still going his way, despite some of the failures of the last year.

The setbacks Carter suffered in his first year in office should be blamed not so much on ill fortune as on presidential errors, most of which can recouped if Carter, a born retriever, puts his keen mind to it. Even his opponents concede that he seldom makes the same mistake twice.

The President's gradual decline in the public-opinion polls is naturally welcome news to the GOP. It is not surprising that Rep. John Rhodes, the aggressive House Minority Leader, should be prompted to say that "it looks like Carter may well be a one-term President."

According to Gallup and Harris, however, nearly all Presidents lose grounds in the polls in their first year in office, but then they almost invariably recover, as Eisenhower did in 1956 when he won in a landslide. Richard Nixon's position in 1971 was so desperate that he devalued the dollar and imposed wage and price controls in peacetime. Yet a year later he carried 49 out of 50 states in winning re-election.

Gerald Ford, who was not an elected President, presided over the nation's worst inflation of the century, along with a steep rise in unemployment. Nevertheless, despite a bitter schism in his own party, he came within 1 per cent or so of retaining the White House, even though he was merely an appointed President.

There is no denying that conditions in the United States could be - and should be - better, such as in health, welfare, employment, cost of living, crime, energy and housing, to name a few items. Nonetheless, the rarely acknowledged but politically significant fact is that on balance most Americans have seldom, if ever, had it better, either at home or abroad.

The year-end figures for the economy are intoxicating , with the final week of Christmas shopping being the biggest in the history of U. S. retailing up 15 per cent or more over 1976. Meanwhile, personal income soared to a record level of $1,597 billion, and corporate profits climbed $8.8 billion for a total of $149 billion in the third quarter.

The American economy in absolute terms is growing faster than any other in the world, with manufacturing exceeding 82 per cent of capacity. Housing in 1977 reached an all-time high of 1.5 million single-home starts, while total starts jumped 23 per cent over 1976.

Most heartening of all, perhaps, is that civilian employment in November rose by 950,000, up 3.9 per cent over November 1976 and the largest monthly gain in 17 years. The increase in jobs for all of 1977 was 3.7 million, for a total of 92.2 million, probably a record percentage of employment among those of working age.

Looking ahead, the consensus of business forecasters if for a still better year in 1978, helped along by labor tranquility and a large tax cut. Recent data, incidentally, show that U. S. taxes already are much lower than taxes in the other major industrial countries.

Internationally, the scene is the most peaceful in decades, with no wars of consequence going on anywhere. Moreover, the future outlook has been brightened by the breakthrough in the Middle East, as well as by the price freeze of the oil-producing nations.

While President Carter can hardly claim credit for all these welcome fortuities, he is bound to benefit ultimately from the public satisfaction they inspired. The most recent Gallup Poll reports that 90 per cent of the American people presently describe themselves as "very happy" or at least "fairly happy."

A series of studies by the Potomac Institute also finds that Americans, despite their grousing, are still "strongly optimistic about their personal prospects and those of their country.

This state of mind may account, in part, for the lackluster record of Congress in 1977. The bland political climate is simply not propitious for radical legislation. Carter, it might be said, has been "plagued" by good times.

He came to office yearning to be a dynamic leader, but how can he demonstrate his talent for crisis-management if there are hardly any emergencies that seem to require urgent and drastic action? We have, it is true, a number of serious problems, but for the most part they are more or less chronic ones that have been with us a long time, and no doubt will be around for years to come.