Washington ? The last week has been full of fulminations over the apparent drop in President Bush’s approval ratings. The White House is worried, the Democrats are hopeful. But that’s not the worst poll news for the president. Bush may remain ahead of John F. Kerry and Joseph I. Lieberman, but he recently dropped behind Dwight Eisenhower.

This is serious. Bush still has a commanding lead over every presumptive Democratic challenger. He’s a wartime president, and he’s struggling to battle tough economic times. Even the Democrats taking on Bush know that they have an uphill battle.

But a recent Zogby International poll of the public’s perceptions of the dozen most recent presidents may be more telling than any other political barometer. Last year’s poll showed that the public believed President Bush was the third-greatest chief executive of the modern era, behind only John F. Kennedy and Franklin D. Roosevelt. This year’s survey shows him at No. 6. Among those who rate higher than Bush this year are Harry S. Truman and Ronald Reagan.

These sorts of polls are easy to dismiss, in part because they represent a national parlor game, in part because the results sometimes seem so silly. Last year, for example, Kennedy ranked as the greatest of the last 12 presidents. Even Kennedy’s fiercest defenders — those who believe JFK faced a serious crisis in Cuba in 1962 and stood, then and now, as an inspiring symbol of American freedom and idealism — would be hard put to argue that a president who served only a thousand days deserves to outrank Roosevelt, who served a dozen years and defeated the Great Depression, Nazi Germany and Imperial Japan.

But the 20-point drop in Bush’s rankings in Zogby’s artificial historical popularity derby does tell us some important things, all of them troubling. It says that Bush, who won bipartisan plaudits for his reaction to the terrorist attacks of Sept. 11, 2001, is not wearing as well in the long term. It says that the public, which recognized the grave danger al-Qaida posed to American freedom, values and safety, is losing its sense of urgency over terrorism. It says that Americans, whose response to terrorism suggested a national conviction that they were living in historic times, have returned to a sense of life-as-usual — and that they believe these times hold less danger for the nation than the conflict in Korea (during Truman’s time), the crises in Hungary and the Suez (in Eisenhower’s era) or the challenges in Central America and Lebanon (in Reagan’s time).

That’s because presidential greatness requires great challenge. We will never know, for example, whether Grover Cleveland had the makings of greatness; he was not president during a time of unusual national peril. Bill Clinton spent much of the latter part of his presidency bemoaning the fact that he never faced the tests that provide the fire for presidential greatness.

Arthur M. Schlesinger conducted a ground-breaking survey of historians in 1948, asking his scholarly colleagues to rate American presidents. He repeated the exercise in 1962 and then, in 1996, his son, Arthur M. Schlesinger Jr., did it again. The last Schlesinger survey identifies only three “great presidents.” They are Abraham Lincoln, who served at a time when the nation literally fell apart; Franklin Roosevelt, who saved both capitalism and democracy; and George Washington, who presided over a country that was not yet a nation.

These surveys illuminate one thing more about presidents’ positions in history — how swiftly is the movement among the rungs of presidential ratings. The Schlesinger surveys bear this out. Thomas Jefferson, for example, rated as a “great” president in both 1948 and 1962. By 1996, he was merely “near great.” His contribution to America’s founding document wasn’t diminished, but his moral standing was; modern Americans are less forgiving than their parents and grandparents of Jefferson’s willingness to hold slaves, and the Jefferson cult has been undermined by DNA evidence proving his sexual liaisons with Sally Hemmings.

Lyndon B. Johnson is perhaps the greatest symbol of presidential revisionism. The 1996 Schlesinger poll ranked him as “average.” Since then, many of his greatest critics, including the economist John Kenneth Galbraith and Vietnam War opponent George S. McGovern, have admitted that they were too swift to condemn Johnson. The LBJ tapes have given Americans new appreciation of the 36th president, and last year a cable TV mini-series portrayed him as downright heroic. The most startling sentence in any paper in 2003 may have appeared in The New York Times last week, when the Rev. Al Sharpton Jr. rated Johnson as “my favorite president.”

Presidents perform for the public but they also audition for history. In his article accompanying the 1996 poll, Arthur Schlesinger Jr. quotes Franklin Roosevelt saying that the nation’s great presidents were “leaders of thought at times when certain ideas in the life of the nation had to be clarified.” This is such a time. The fact that the public places Bush behind Kennedy, Truman, Eisenhower and Reagan — who presided over hard times, to be sure, but not times of profound challenge to the nation’s values and institutions — is dangerous for the president and is dangerous for the nation.