The American Presidency; Never set in stone, it has proved flexible yet enduring

America goes through a brief interval of patriotic introspection every year at the joint celebration of Washington-Lincoln birthdays. This time it comes just after the anniversary of Franklin Roosevelt. The occasion brings up annually the question of what the United States is all about, and how it got that way.

The concept of the American presidency and its accompanying problems has caused debate for nearly 200 years. It has baffled many, including some Americans. The office didn't turn out quite the way it was supposed to. The presidency sometimes gives a bumpy ride but stays aloft. What keeps it airborne, as part of the unique US tri-motor separation of powers, is disputed. Now in the Washington-Lincoln birthday celebrations, Americans are giving the presidency another of their quick, affectionate, mystified glances - it is unlike any other democratic system in the world.

The unique feature of the government is its separation of powers. This triangular relationship is not static and the balance constantly varies between White House, Congress, and the courts. These varying changes would startle George Washington, Abraham Lincoln, or even Franklin Roosevelt. When FDR took office, the dominant occupation in America was agriculture; now farmers make up only around 6 percent of the population. Again, the racial segregation that occurred in parts of the country in Roosevelt's first years has been removed. Another change is the relationship of government to poverty: European nations have acknowledged social responsibility for the poor and unemployed since Bismarck, but not till the 1929 stock market crash and resulting depression did the idea leap the Atlantic. It has now grown to a point where it has produced a counterrevolution under President Reagan. The Founding Fathers would rub their eyes. They invented the most radical government of their time, but some still believed in rule by ''the rich and well born.'' Their means for selecting presidents has been debated ever since.

Lord Bryce wrote a famous chapter on the American government: ''Why great men are not chosen presidents.'' He observed: ''The ordinary American voter does not object to mediocrity. . . . He likes his candidate to be sensible, vigorous, and above all what he calls 'magnetic,' and does not value, because he sees no need for, originality or profundity, a fine culture or a wide knowledge.''

Seventy-five years later Time magazine asked its readers (Dec. 15, 1976), ''Can anyone remember when he last went to vote for a US president and felt both enthusiastic and confident?'' After going through some of the candidates, it asked, ''Out of our large (214 million) and highly educated population, is this the best choice the American system can offer?''

What the political parties must look out for is not a good president but a good candidate, Mr. Bryce argued. Two political scientists, William R. Keech and Donald R. Matthews, of the Brookings Institution, have more recently declared:

''The United States has the most elaborate, complex, and prolonged formal system of nominating candidates for chief executive in the world.'' It runs from selection of delegates to the nominating conventions through the primaries.

Why doesn't the electoral system produce more George Washingtons and Abraham Lincolns, it is sometimes asked. Today the public seems to show restlessness; opinion polls note uncertainty and questioning. In the 1980 presidential election some 46 percent of the eligible voters didn't bother to go to the polls. The number of voter dropouts has increased steadily since 1960. In recent elections the stay-at-homes theoretically could have changed the result.

The revolutionaries who helped devise the Constitution did not regard it as something that wouldn't change in time: It has never been static. George Washington was pleased with the new instrument and wrote to the Marquis de Lafayette on June 19, 1788, while ratification was still pending, that he expected ''many blessings will be attributed to our new government'' that would actually flow not from it but from the virtues and frugality of the people themselves:

''When the people shall find themselves secure under an energetic government, when foreign nations shall be disposed to give us equal advantages in commerce from dread of retaliation, when the burden of war shall be in a manner done away by the sale of western lands, when the seeds of happiness which are sown here shall begin to expand themselves, and when everyone (under his own vine and fig-tree) shall begin to taste the fruits of freedom, then all these blessings (for all these blessings will come) will be referred to the fostering influence of the new government. Whereas many causes will have conspired to produce them.''

Washington clearly saw the difference between the Constitution as an instrument and people who produced it.

Abraham Lincoln showed the extraordinary capacity of the government to expand its powers in an emergency. Nobody doubts that these war powers are still available for better or worse if the need arises, or if a man strong enough to exercise them has the support of the public. If Lincoln had not done as he did, the Union might very well have been lost on April 15, 1861. The President on his own authority called for 75,000 volunteers. He enlarged the Army and Navy, commandered the railway between Washington and Baltimore, and eventually to Boston, and established conscription. He also suspended the writ of habeas corpus for all persons suspected of ''disloyal practices.'' Various writers have discussed these actions, and it is noted that they were ultimately ratified by Congress.

The elasticity of presidential power under the American system has worried some observers. George E. Reedy, special assistant to Lyndon B. Johnson, wrote in his book, ''The Twilight of the Presidency'' (1970), of possible deadlock. The outcome, he gloomily forecast, might be a presidential choice between chaos and suppression of dissent. Few writers, however, carry it to that extreme.

But Thomas E. Cronin, in his ''State of the Presidency'' (1980) says: ''Presidents have concentrated in recent years on selected areas of the presidential job. In part, this is because we have created a nearly impossible presidential job description. . . . In part, however, recent presidents have also been, or so it would appear, lulled into responding to those parts of their job that are more glamorous, more prominent.''

A cycle runs through recent presidencies. There is the initial euphoria, winning the election, picking a Cabinet, enunciating plans. After the first year , a different reality sets in. There is the nitty-gritty of the economy. Congress must be courted or threatened. Both Jimmy Carter and Ronald Reagan, it is noted, were ''outsiders,'' who came to power ''against'' Washington. It sometimes happens that when presidents are frustrated at home, they turn to foreign affairs in which they are less limited by Congress.

Neither Mr. Carter nor Mr. Reagan had served in Washington, though each had gone through the gubernatorial training that often is the preparation for American presidencies. Richard Rose, who writes on comparative politics, speaks of the contrasting European system: ''To become prime minister, a politician must spend years in the service of the party in Parliament. Since 1945 (he wrote in 1980) the length of the average parliamentary apprenticeship of a newly installed incumbent has been 28 years. A prime minister gains office in competition against colleagues by building up expectations about how well he or she can do the job. A prime minister holds office only as long as he or she retains the confidence of . . . the governing party.''

Mr. Rose might have made the point that Winston Churchill, who became prime minister in 1940, was first elected to Parliament in 1900.

The discussion of the American presidential institution never ceases, perhaps stimulated by the Watergate scandal. Historian Arthur M. Schlesinger wrote a provocative book, ''The Imperial Presidency,'' arguing that presidential power became so expanded and abused by 1972 that it threatened the constitutional system.

In a brilliant survey in 1980, James L. Sundquist of the Brookings Institution notes that voters often blame presidents for problems that are not personal but institutional. ''One can identify only a few periods in the entire 20th century when relations were close enough or presidential leadership strong enough . . . to achieve major innovations in controversial areas of public policy,'' he said. ''The most notable of these were the first two years of Woodrow Wilson's administration, when the New Freedom was enacted; the first term of Franklin D. Roosevelt, when the New Deal took form; and the first two years of Lyndon Johnson, when the Great Society was founded. Each of these . . . active intervals came after a massive presidential (election) landslide which established the president's credentials as a leader. . . .''

President Carter called it ''malaise,'' but Mr. Sundquist says he thinks it is a ''crisis of competence in our national government.'' Based on opinion polls , he argues that there has been a precipitous decline in confidence since the mid-1960s. Since Sundquist wrote, Mr. Reagan has taken office, but he seems to be subject to the same pulling and hauling as his predecessors. A sign of more difficult days is that President Eisenhower, who was elected in 1952, is the last executive to serve two full terms.

It is the strong presidents, Washington, Jefferson, Jackson, Lincoln, the two Roosevelts, and Wilson, that Americans celebrate. Who remembers those who were content to preside over the executive branch while letting Congress steer? The US is almost unique in the number of political appointees placed at the head of executive departments. Efforts are made recurrently to increase the corps of career administrators. They run counter to a tradition of government by amateurs in Washington that goes back to the time of Jackson.''

What is necessary, first of all,'' Sundquist says, ''is that those who decry the shortcomings of governmental performance recognize that the fault does not necessarily lie in the individuals who may happen to occupy the White House or sit in Congress, and that replacing them with other individuals will not necessarily help. Severe institutional and structural problems must be addressed.

''We don't like to think of inadequacies of government when we are celebrating the birthdays of our greatest men, Washington and Lincoln. Looking out over the land the calm figure of Lincoln sits there in his marble memorial and reminds America of its aspirations and victories, not of its failures. In the same capital, the noble Washington Monument rises up so high that you can almost see Mount Vernon from it. A volume just published by historian James MacGregor Burns (''The Vineyard of Liberty'') reminds us of Washington, who presided in the long, hot summer of 1787 in Philadelphia:''

His magisterial appearance, his clothes and equipage, his superb Presence in ceremonial situations, and above all, his ability to seem to hover far above the squalid politics of party and parochialism - all these delighted many Americans and alarmed others. . . .

''Washington wanted government - especially this new, frail, vulnerable government - to be respectable and dignified. It was for this reason, and because he appeared increasingly to enjoy display for its own sake, that he drove about Manhattan with a carriage and six cream-colored horses; kept 14 white servants and seven slaves in his house on Broadway, put on large and elaborate dinners. . . . Occasionally he rode about town on a white steed with leopard-skin housing and saddlecloth bound in gold.''

But we are apt to forget another side of this unapproachable figure. We know his wife as ''Martha'' - apparently he didn't. In a letter dated June 18, 1775, recently in the New York Times, he wrote of his appointment to the Supreme Command of the Revolutionary Army. He says:

''You may believe me, my dear Patsy, when I assure you in the most solemn manner, that, so far from seeking the appointment, I have used every effort in my power to avoid it. . . .'' He repeats it at the close of the letter, which he ends touchingly, ''And to assure you that I am, with the most unfeigned regard, my dear Patsy, your affectionate George Washington.'' Martha to the nation. ''Patsy'' to the Father of his country.