The United States might have conquered the Depression and won World War II if an errant polio virus hadn't struck Franklin Delano Roosevelt after a swim in the Bay of Fundy, but the moral strength FDR drew from his physical weakness fortified the country at perhaps the most perilous period of the last century. The nation might have built the Panama Canal, assembled the Great White Fleet, and become a muscular 20th century power if Theodore Roosevelt hadn't fought to overcome a sickly youth, but the determination that TR brought to life was also the determination that America brought to its first star turn on the world stage.

It is, to be sure, dangerous business to transform a politician's struggle with adversity into a Rosetta stone of his character. But it may be even more dangerous to ignore those struggles. A Thomas Jefferson who wasn't an early widower; an Andrew Jackson who wasn't scarred by the slash of a British officer's sword across his face, by two bullets that he carried in his body for years, and by the ``corrupt bargain'' that denied him the presidency in his first campaign; an Abraham Lincoln who didn't suffer from depression; a John F. Kennedy whose PT boat wasn't sunk in the Pacific; a Lyndon Johnson who hadn't been reared in hardscrabble Texas and who hadn't taught school to poor Mexican-Americans - all of them would have been immensely different men from the people who eventually ascended to the presidency. As a result, this would have been an immensely different country.

And so here is a notion that turns intuition on its head: Seldom courted, never fully avoided, adversity is an advantage. At least in politics, and probably in life, too.

Great turning points - George W. Bush's struggle with a midlife crisis in Midland, Texas, for example, or Bill Bradley's humiliating performance in his first year in professional basketball - are part of what John Adams, speaking a decade before the Declaration of Independence, called ``that most dreaded and envied kind of knowledge - I mean of the character and conduct of [our] rulers.''

Americans, who are, after all, the ultimate arbiters in American politics, are drawn naturally to the figures who have overcome adversity. That is why as early as the campaign of 1840, William Henry Harrison, who was born on an estate in Virginia and lived in a 22-room manor in Ohio, allowed his Whig campaigners to shout from the rooftops that he had been born in a log cabin. Herbert Hoover, who nearly died of the croup at age 2 and was permanently scarred when he stepped on a hot iron chip in his father's Iowa blacksmith shop, became one of the most appealing figures of the early 20th century because of his rise from a hard childhood to the giddiest heights of engineering and business.

Indeed, until the advent of the Bush clan in presidential politics, the Republicans, beginning with Lincoln, often chose nominees of the striving classes while the Democrats, whose populist heroes include FDR and JFK, often displayed a weakness for well-bred if not aristocratic candidates.

But often it is adult adversity more than childhood circumstances that gives shape to a politician's attitudes, approach, and view of the world. Richard Nixon, whose family struggled amid the pleasant orange groves of southern California, campaigned in 1960 as the candidate of experience, but in truth it may have been the experience of the 1960 campaign that changed him most. A man of extraordinary accomplishment - he had been elected to the House, the Senate, and the vice presidency by the time he was 40 - his narrow loss to Kennedy under terms he considered corrupt embittered him and persuaded him that any political tactic was legitimate.

Though Theodore Roosevelt's early adversity hardened him, later adversity softened him. By 25 he was a widower, with his beloved wife, Alice, the Valentine's Day victim of Bright's disease. ``We spent three years of happiness such as rarely comes to man or woman,'' Roosevelt wrote in his memorial to his wife, adding: ``As a flower she grew, and as a fair young flower she died. Her life had always been in the sunshine.... And when my heart's dearest died, the light went from my life forever.''

Unexpected and tragic deaths changed the lives of future presidents, as they do of the rest of us. The death of Earl Carter at 59 ended the promising Navy career of Jimmy Carter, prompting him to resign his commission, return to Plains, and abandon his dream of traveling the world and living the sort of cosmopolitan life that was unavailable in Georgia peanut country. The death of his 4-year-old daughter, Robin, from leukemia in 1953, was a jolt to George Bush and a tragic reminder that no life, even his, is charmed forever, or for long.

Deaths of children during their presidencies dramatically altered the equilibrium of both Lincoln and Calvin Coolidge. Willie Lincoln's death in 1862 devastated his father and broke his mother emotionally. Calvin Coolidge Jr. died in 1924 from a foot infection traced to a blister he developed during a game of tennis. ``When he went,'' Coolidge said, ``the power and the glory of the presidency went with him.''

And, in Harry Truman's case, failure in business almost certainly contributed to success in politics. His haberdashery fell victim to the recession of 1922. That November Truman was elected judge for the eastern district of Jackson County and four years later he was elected presiding judge of the county. In his first months in the presidency, Truman stated, ``There'll be no more to this job than there was to running Jackson County and not any more worry.'' Not true, of course, but, still, Truman was onto something. The lessons he learned in the past - the raw materials of that most dreaded and envied kind of knowledge - would help him navigate the future, and change ours.