South Bend, Ind. ? For someone who looks and sounds far different from any previous American president, Barack Obama sure invites a lot of comparisons with his predecessors.

First the Illinois senator announced his candidacy at the Old State Capitol in Springfield, where Abraham Lincoln delivered his “House Divided” speech in 1858. He has drawn on Lincoln comparisons and — not a surprise for the first black president — installed Lincolniana throughout his White House.

Then Time magazine portrayed Obama on its cover as a modern-day Franklin Delano Roosevelt, complete with hat and cigarette holder at jaunty angles. Last month the conservative Weekly Standard’s cover showed the president, in classical pose, contemplating a bust of Jimmy Carter. Not long ago The Wall Street Journal carried a lengthy piece comparing the (scant) foreign-policy experience of Obama and Harry S Truman.

And all along we were told this was a presidency beyond compare.

What is it about this president that invites so many allusions to other presidents? Nobody did that when John F. Kennedy or Ronald Reagan entered office, although both were widely imitated after they left the White House. Is it an extension of the way that so many voters last fall projected their own yearnings onto a relatively unknown figure, believing that their perspectives were his perspectives and their hopes were his hopes? Is it a lack of imagination, or flawed historical literacy, among commentators?

The truth is that all of these comparisons have limited utility. There have been only 43 presidents (we won’t count Grover Cleveland twice) and each has governed in a decidedly different time.

What Lincoln faced as the Union was tumbling apart is a lot different from what Obama is facing at a time of partisan tension. What FDR faced as capitalism itself was under siege in the Great Depression is a lot different from what Obama is facing at a time when, despite the terrible toil of what we might call the Great Recession, most Americans still are reasonably well-off and have at least moderately bright prospects. By the same token, what Obama is facing, with economic distress and two wars raging, is far more perilous than what William Howard Taft faced exactly a century ago.

It is a mistake to reach for the familiar to try to explain the present. “American presidents are different, this one especially,” former German Chancellor Gerhard Schroeder said in a conversation the other day. “His pure existence changes not only the way the world views the United States, but the way we view the American presidency.”

Indeed, there are flaws with all of these comparisons — flaws that exaggerate the president’s failings or overstate the president’s challenges or warp our notions of past presidents. Let’s examine the most prominent ones:

The Lincoln comparison

The 16th president faced the gravest peril presented to any chief executive in history. (George Washington and Franklin Roosevelt arguably tie for second place.) The survival of the nation was at stake; indeed, the nation had split apart by the time Lincoln took his oath of office, with seven states departed by the time the Lincoln era began. Huge economic, social and cultural questions required instant resolution, with the result that the United States instantly became an industrial nation committed to racial equality, though it would take decades for the country to reach full industrialization or to approach full equality.

Today we are in interesting, vital and important times. But, at least so far, the stakes are not nearly as high as they were between 1861 and 1865.

The FDR comparison

The 32nd president faced an economic crisis that was deeper and two wars (one in Asia, the other in Europe and North Africa) that held far more peril for the future of the United States than the current economic crisis and the conflicts in Iraq and Afghanistan — although the advent of a nuclear al-Qaida could change that balance. When the Roosevelt years ended, the nation was comprehensively changed, in economic policy, in global profile and in political philosophy. For all the promise of the Obama era, nothing quite so profound seems likely now, even in health care.

The Truman comparison

Like Obama, Truman came to office from the Senate and with little experience. Like Obama, he acquired strife in stereo — a war that still required mopping up in Europe and one that required difficult practical and moral questions in the Pacific. Then, in an instant, he had to face the threat from the Communist ascendancy in Europe and Asia.

Truman possessed less formal education and perhaps more native smarts than Obama, though the same could be said about any president since Truman and sometimes is. The big difference between these two eras is the confidence factor. Though engaged in a bipolar struggle with the Soviet Union, the United States was strong and bold in the Truman era. Though the single superpower in the Obama years, the nation is tentative and wary today.

The Carter comparison

It is hard to remember this, but the 1976 election, which brought to power the first Southern president since the Civil War (Lyndon Johnson, who assumed the White House after Kennedy’s death, doesn’t count), was regarded as a dramatic turning point in American history. So, too, for racial reasons, was the election of Obama a year ago this month.

Though the country faced a crisis of spirit in both administrations, the challenges Obama faces are far graver than the ones Carter inherited, and his willingness to project strength is greater than that of Carter, who possessed military bona fides that Obama lacks.

At his Notre Dame commencement address in 1977, Carter spoke of an “inordinate fear of communism.” At a Notre Dame commencement a third of a century later, Obama did not speak of an inordinate fear of terrorism but instead talked of the threat from “those who will stop at nothing to do us harm” and warned of what might happen “when weapons in the hands of a few can destroy the many.”

The truth is that while his critics might not consider Obama to be his own man, in history he will be regarded that way — a unique president for a unique time. He may be considered uniquely weak, or uniquely masterly, or uniquely unsuccessful, or uniquely visionary. But critics and supporters alike should recognize this about Obama, or any other president: There’s nobody quite like him.