YOU DO NOT NEED to be a historian to observe that presidencies of Harry Truman and Gerald Ford contain many parallels beyond the circumstance that both men were vice presidents before their assumption of office.

Historical comparisons of the two leaders come naturally mind in reading John Hersey's new book, Aspects of the Presidency, which reprints the author's New Yorker articles of 1950 and 1951 on Truman and the New York Times Magazine article of 1975 on Ford, later that same year made into a book, The President. In preparation, Hersey spent a week with each man, 25 years apart, as a most privileged observer in and around the Oval Office.

Besides confronting unpopular and unwinnable Asian wars, Truman and Ford both had serious problems with Capitol Hill. Truman won reelection after attacking the "do-nothing" 80th Congress which he had previously failed to exhort to legislative action. Ford spent most of his 28 months in office struggleing to restain the large plans of the big Democratic majorities in Congress.

Truman and Ford were both certified underdogs when they sought election to the White House in their own right. Author Hersey, during the 1948 campaign, took two of his sons, then aged 7 and 5, to the Norwalk, Connecticut, railroad station to hear Harry "give 'em hell," which, of course, Truman did that day. He did it again on election day, in spades, to the smugly overconfident Republican ticket of Governor Thomas E. Dewey and Governor Earl Warren.

Jerry Ford lost the presidency to former governor Jimmy Carter, but the outcome does not make Ford's comeback any less remarkable than Truman's, only less successful. At the end of August 1976, Ford trailed Carter by at least 25 percentage points in all syndicated public opinion surveys. In American presidential elections, one percentage point represents 800,000 votes, Exactly 11 weeks before election day, Carter led Ford by 20 million votes. A switch of only 25,000 voters' minds in Ohio and Mississippi and President Jerry Ford, not Harry Truman, would be the inspiration of every political underdog.

Hersey himself abstains from any actual comparison of the two men or their presidencies. But Hersey's trip to the Norwalk station with his sons in tow is instructive. John Hersey was a fan of Harry Truman and he manages to provide the reader with sufficient personal testimony to more than substantiate his admiration, if not his total affection, for Truman. m

Hersey's White House Truman is thoroughly likable in large part because the man from Missouri was so thoroughly American. Truman was remarkable egalitarian, a president who knew personally not only his secret service agent but the agent's child. He was ready to laugh at all manner of things creaters, including most especially the 33rd president, and he is hopelessly curious.

Harry Truman knew that Teddy Roosevelt built the West Wing of the White House, Herbert Hoover built the East Wing and Calvin Coolidge literally raised the White House roof. But much more important, Harry Truman knew exactly who he was. Apparently he liked himself as much as he liked the political profession he practiced so vigorously. Truman on politics: "You know people cuss the politicians all the time, but how do you think this country would get along without the honest politicians . . . my definition of a politician: 'a politician is the ablest man in government, and after he dies they call him a statesman.'" In our joyless era of "citizen-candidates," who emphasize their absence of political skills and alliances as evidence of their alleged moral superiority, it is refreshing to hear Truman on his vocation and its indispensability to the survival of the republic.

Hersey has trouble with Jerry Ford, perhaps because he subscribes to the thesis that more compassionate congressmen vote more funds for social programs and less compassionate congressmen vote against these same programs. The problem is that Hersey likes Ford very much, which is understandable, because Hersey's Ford is considerate, unpretentious, serious, mentally healthy and absolutely without bitterness -- Ford with any sort of an "enemies list" is unimaginable. w

Hersey's "problem" with Ford is that the president -- who Hersey himself represents an an exceptionally patient man, and kind to subordinates -- did not vote as a congressman for all the "good people programs."

This is the obvious shortcoming of the author's absence from the scene as a reporter or writer since 1975. Even Hersey must recognize by now that double-digit inflation has been and continues to be for Americans born after the beginning of the New Deal a seminal economic and social experience. In fact, inflation is -- for the present generation -- what the Depression was for an earlier generation, requiring its victims to change the way they think about themselves, their obligations to the community and the future. Hersey never acknowledges that Jerry Ford may have feared the growth and permanence of inflation.

What Ford did not communicate to Hersey was any coherent vision of the society and the nation he sought to lead. This failure to communicate a vision probably more than any other factor cost Jerry Ford the 1976 election.