ASHINGTON - Political scientists, psychologists, and sociologists spend endless hours unlocking the mystery of how, and why, voters change their minds. Meanwhile, historians are changing their minds about the choices voters have made already.
For that reason, in 50 years Harry Truman has gone from a rube to a prophet, and Dwight Eisenhower has been transformed twice, from military hero to snoozy caretaker to - the latest interpretation - canny politician. And though the balladeers who sing tributes to John F. Kennedy like to play off the Irish song ''Johnny, we hardly knew ye,'' the more accurate remark, about Kennedy and others, goes something like this:
Johnny, we hardly recognize ye.
Journalism consists of history written on the run, but the men and women who sit, think, and write serious history often come to vastly different conclusions. And so, too, does the public, which is never reluctant to alter the pencil sketches that comprise its impressions of presidents.
One reason, of course, is new material that rises, like ground water, to the surface over time - or fresh archival documents that are released years after the first drafts of history are written.
Many of those documents become available because complicated legal strictures have expired (typical example: '' Nothing may be released until 50 years after the death of all the principals'') or after archivists have finished processing them (typical example: the tapes of President Johnson's first months in office, which were transcribed meticulously and annotated insightfully by Michael Beschloss before they were released, beginning only in 1997).
But more than the geology of history - the raw materials - changes over time. The climate of history changes, too.
World War I, for example, has always been regarded as a dreadful conflict produced by misunderstandings and misreadings by leaders who stumbled into war. But in recent years, historians have been willing to consider that the beginning of the Great War might have been produced, or at least abetted, by the cold calculations of clear-thinking men, some in London, others in Berlin. The Vietnam War, first considered a pure product of Cold War tensions, then a tragedy of hubris and human error, is undergoing a brisk revisionism of its own, with some modern thinkers willing to reconsider the causes and results of the war - and willing to conclude that the conflict might have been far more noble than the conventional consensus acknowledges.
These wars are part of the backdrop of American presidential elections, and as views about the conflicts are altered, so, too, are views about the leaders who prosecuted the wars - or who succeeded wartime presidents.
Woodrow Wilson, the preacher's son and university president who campaigned for reelection in 1916 on the basis of having kept the United States out of war, led the nation into war only a few months later. Just as Wilson changed his mind, history's view of Wilson has changed, too. Where once Wilson was a romantic idealist, now he is regarded as a stubborn ideologue. (Such conclusions often say as much about the contemporary period as about the past. Today, when ideology is out of fashion and the two parties are about to nominate moderates for president, ideologues are regarded as stubborn rather than as courageous.)
In this year's campaign, the Democratic candidate, Vice President Al Gore, is sometimes denigrated as a stern intellectual with effete interests such as the environment, nuclear deterrence, and government organization. If he is elected and the times call for insight on those issues, he may be rewarded by history. The Republican candidate, Governor George W. Bush of Texas, sometimes is dismissed for his easygoing personality. If he's elected and his bedside manner ministers the nation through a crisis impossible to foresee today, he will win points in the history books.
But the transformation, for example, of President Nixon from cold-blooded Cold Warrior to stumbling trickster to coldly calculating miscreant and, more recently, to diplomatic magus reflects more than the passage of time. It reflects a change in perspective. So, too, does the change in Johnson's image from small-minded Vietnam commander to big-hearted social visionary.
Few historians would argue that Johnson succeeded in Vietnam, and the sober roll call of the dead on the walls of the Vietnam memorial in Washington, D.C., is testimony to the price the nation paid for the conflict. Even so, Johnson's reputation is experiencing something of a revival, even in a period that is skeptical of the sort of big-government social programs that were at the center of the Great Society.
In his monumental 1961 Trevelyan lectures on the nature of history, E. H. Carr, who was both a journalist (assistant editor of The Times of London during the World War II years) and a historian (fellow of Trinity College, Cambridge, and author of numerous volumes on Soviet Russia), explained how the view of history shifts:
''Pregnant failures are not unknown in history. History recognizes what I may call `delayed achievement': the apparent failures of today may turn out to have made a vital contribution to the achievement of tomorrow.''
Carr argues, moreover, that the historical verdict may be shaped by ''things that have not yet happened.'' The eventual triumph of the civil rights movement, for example, put both Kennedy and Johnson on the right side of history - and besmirches the presidential reputation of Eisenhower, whose enthusiasm for the 1954 Supreme Court school-desegregation case was, at best, muted.
President Clinton often is lost in reveries about his own place in history, and he is given to speculate that he may eventually find a place in the upper-middle ranks of presidents, not in the same category as Franklin Delano Roosevelt and Abraham Lincoln (the reigning champions) but neither in the same category as Andrew Johnson (the only other president to be impeached, and universally regarded among the worst American presidents).
Clinton takes solace in the resurgence in the reputation of Truman, who is revered today but who never approached cult status in his White House days, when the byword sometimes was ''to err is Truman. '' Today, however, he is regarded as an inspirational figure, often for the very same reasons he was denigrated while in office: his earthy language (now hailed as plain-spoken honesty), his plodding approach to problems (now celebrated as the American work ethic made incarnate), his stubbornness (now saluted as an emblem of his integrity), and his reliance on intuition (now regarded as proof that he was comfortable with himself).
It is impossible to divine how history may regard some of the familiar recent presidents. Will Ronald Reagan, for example, be rewarded for altering Americans' views of government or punished for the deficits he ran up? Will George Bush be celebrated for winning the Gulf War or for failing to invade Baghdad and topple Saddam Hussein?
But the biggest mystery may be Clinton. The president's job-approval rating from historians will be affected by the economy, by his affair with Monica S. Lewinsky, by his impeachment in the US House of Representatives and acquittal by the Senate, even by whether Gore is elected to succeed him. But it will also depend on events that will transpire after he leaves office. The verdict on his efforts in the Middle East (where he sought peace) and the Balkans (where he was willing to go to war), for example, almost surely will be affected by whether those regions experience peace or war in the next several years.
The Oxford historian Geoffrey Barraclough once traced how the interpretation of 19th-century Europe evolved in the 20th century. ''The historian who takes his stand not in 1815 but in the present will see the same period in different proportions,'' he wrote. ''His starting-point will be the global system of international politics in which we live today and his main concern will be to explain how it arose.''
And thus the ultimate irony of history: The past can be redefined by the future. It turns out that the portrait of a president is not a stilllife.
