After seven months of brilliant, indeed heroic, presidential leadership, President Bush's behavior after the gulf war - his weak and vacillating hands-off, I'm-out-of-here policy - is a puzzle.

The best explanation I have yet heard is this: Bush was like the man who wins the jackpot in a casino and walks right out the front door refusing even to look at another table. (To which one wit added: Bush then gets mugged getting into his car.)There are many reasons, of course, why Bush decided to cash his chips even if that meant abandoning the Iraqi rebels to Saddam Hussein's tender mercies. (That policy was partly reversed when the extent of the Kurdish catastrophe became clear.)

There was the fear of getting dragged into an inconclusive civil war, a belief that international law and the wartime coalition would support saving Kuwait's sovereignty but not violating Iraq's, and his susceptibility to pressure from his Saudi friends who feared both the fracturing and democratization of Iraq.

These were all factors, but the overwhelming one was the president's persona: A man of pathological prudence, having just risked everything on one principled roll of the dice, was not about to hang around the gaming room a second longer. Bush had won the war and saw no reason to risk its political fruits - his 90 percent standing in the polls was a new indoor record - on a problematic involvement in Iraq.

It was a question of political capital. After 30 years in politics Bush had finally amassed it. He was not about to spend it in Kurdistan.

Political capital means a lot to this administration. It has long been exquisitely sensitive to its standing in the polls. This is an administration whose secretary of state once responded to a policy criticism from the Senate Majority Leader Mitchell (who had accused the administration of timidity and nostalgia for the Cold War) with the smug, "When the president of the United States is rocking along with a 70 percent approval rating on his handling of foreign policy, if I were the leader of the opposition party, I might have something similar to say."

The willingness to risk political capital is not just a sign of greatness in a leader, it is almost a definition of it. Take two cases, democratic leaders who, in the past year, have been resoundingly rejected by their people. Margaret Thatcher was summarily dismissed from the prime ministership, her approval rating at a historically low 29 percent. And in Poland's presidential election, Prime Minister Tadeusz Mazowiecki came in third with 18 percent, behind a right-wing kook from Canada among whose goals for Poland was a hundred medium-range nuclear weapons.

Both defeats were a reward for toil. Thatcher had spent a decade systematically dismantling whatever parts of Britain's suffocating socialist state she could get her hands on. Mazowiecki had just put through arguably the most heroic economic reform of the postwar era.

Both leaders had simply spent all their political capital doing the unpopular but necessary. Both left defeated. Yet history will undoubtedly rank both highly, as instrumental to the revival of their countries.

Contrast this with our most popular recent president. Ronald Reagan left office with the highest approval ratings since World War II. Like dying rich, this is a great moral failure. It should be a rule that any president who leaves office with high approval ratings be automatically disqualified from considerations of greatness.

There are many things on which Reagan could have, should have expended political capital. He never made Americans aware of the costs either of prosperity or of world dominance. He gave us low taxes and a vast military buildup - for which it won the gulf war for us - but at no apparent cost.

The one thing he resolutely declined to do throughout his presidency was to utter the word sacrifice. He was rewarded with applause. Applause fades.

And contrast this with Harry Truman, who left office as the most despised president of our time. (Counting only those who left office voluntarily: upon resignation, Nixon ranked lower.) Yet Truman is remembered as one of the great 20th century presidents for having built, with little public enthusiasm, the enduring structure of the postwar West.

Spending one's popularity is a mark of greatness. (Though not definitive proof: It can be merely dissipated, as occurred during the Carter presidency.) And having had his brush with greatness, Bush would rather not risk it again.

For seven months he was highly imprudent. He put his presidency on the line in a venture that, particularly in the early days of August, did not look at all promising. He not only expended political capital. Having never acquired the kind of personal following that Reagan had, he lived on borrowed capital. In effect, he mortgaged his presidency to success in the gulf. And with success, his immediate instinct was to head for the bank. He did not pass Go.

He has six years left in which to spend his political winnings. He decided not to spend any of them on reshaping Iraq. Fine. Let's hope he finds something worthy - say, an energy tax, gun control, SDI or some visionary scientific venture - to spend it on. Unpopularity is not proof of a great presidency. But it is a good sign.