Accepting responsibility is an essential part of everyday life, something every parent and child, every boss and worker, every friend and colleague wrestle with, or know they should. But for a president it is quite rare, and at least in the view of some historians and government experts, getting rarer, as a national culture of shifting blame permeates American politics.

So it was last week that some powerful words were spoken to the spouses and families of those who died two and a half years ago in the terror of Sept. 11.

"Your government failed you. Those entrusted with protecting you failed you. And I failed you." The words of apology were unmistakable, but the face was hard to place. It belonged to none of the recognizable leaders of the government - not President Bush or Defense Secretary Rumsfeld, Secretary of State Powell or Condoleezza Rice, the national security adviser. Here was a middle-aged man with disappearing white hair and an American flag pinned in his left lapel: a former middle-level foreign policy official of three presidential administrations named Richard A. Clarke.

"We tried hard," Mr. Clarke told the families as he testified to a commission looking into Sept. 11. "But that doesn't matter because we failed. And for that failure, I would ask, once all the facts are out, for your understanding and for your forgiveness."

The mea culpa appeared deeply meaningful to the bereaved families, who thronged around Mr. Clarke when he completed his testimony. But President Bush offered no similar statement, nor did Bill Clinton, for whom Mr. Clarke had also worked.

It is one thing for a deputy at the National Security Council to accept blame on behalf of not one but several administrations, an act perched between admirable and presumptuous. But it is quite something else for a president of the United States to say he is sorry.

In October 1983, terrorists in Lebanon drove a truckload of explosives into a building housing American marines, killing 241. That December, a Defense Department commission prepared to release a report castigating officers in the chain of command for failing to safeguard their troops.

A copy was sent to President Reagan before its release. He read through it, David R. Gergen, then an aide, recalled, and with little discussion headed for the press room. "If there is to be blame," Mr. Reagan said before the assembled corps, "it properly rests here in this office and with this president. And I accept responsibility for the bad as well as the good."

The commanders, Mr. Reagan said, should not be punished "for not fully comprehending the nature of today's terrorist threat."

There was some criticism at the time that Mr. Reagan had pre-empted the military disciplinary process. But over all, Mr. Gergen said, the acceptance of responsibility for something that happened during his term vastly improved Mr. Reagan's status with the military and strengthened him for the rest of his presidency.

"Every time I've seen a president or his team take responsibility it has had a salutary effect," Mr. Gergen said. "The reason why it has become so rare is the way the blame game is played. It can be so ferocious that any time they admit the slightest mistake it's going to be exploited by the other side."

Of course, accepting responsibility, let alone blame, for the events of Sept. 11 is on a scale different from virtually anything else a modern president has had to deal with. Certainly, an argument could be made that Sept. 11 is more analogous to Pearl Harbor than to Beirut, and Franklin D. Roosevelt never accepted responsibility for that sneak attack. Indeed, he talked the Republicans out of making it an issue in the 1944 campaign, saying it would hurt the war effort.

Within hours after the World Trade Center towers crumbled, Bush and Clinton partisans began blaming each other for the failure to stop Al Qaeda, and have been doing so ever since in any venue they can find.

The record is actually surprisingly clear, that there was a series of moments stretching back from Sept. 11 across at least eight years when more aggressive actions might have produced a different outcome that crisp, blue morning. For example:

In 1997 a commission led by Vice President Al Gore recommended steps to tighten airline security, including tougher screening of passengers and stronger locks on cockpit doors. Civil libertarians and the airline industry resisted.

Osama bin Laden, while hardly a household name, was well known as a threat. (Indeed, this newspaper ran a front-page series about him just as the Bush administration was entering office.)

The World Trade Center was already clearly marked as a target, from the bombing in 1993, and the idea to use planes as missiles was known from a disrupted plot to bring down the Eiffel Tower.

So who is responsible for not putting all this together, for failing to avert the tragedy? The airline industry? The Central Intelligence Agency? Richard Clarke? Mr. Bush? Mr. Clinton?

The most famous presidential keepsake in American history is arguably a 2½-by-13-inch glass sign made at the Federal Reformatory at El Reno, Okla. On one side, the side that faced the president, it said, "I'm from Missouri." The other side, the side that faced visitors to the Oval Office, said, "The Buck Stops Here."

To Harry S. Truman that meant accepting responsibility for making tough decisions, including firing Gen. Douglas MacArthur. But it did not necessarily mean expressing regret for them later. He was proud of saying he never lost sleep over his decision to drop the atom bomb, and 10 years later when he was invited to Japan he said he would go only if he did not have to kiss the posterior portion of any Japanese citizen's anatomy. (He didn't go.)

Mr. Bush made it clear last week that he was more in the Roosevelt than the Reagan mode of the responsible commander in chief, offering a narrow test of presidential responsibility in the Sept. 11 context.

"Had I known," President Bush said the day after Mr. Clarke's testimony, "that the enemy was going to use airplanes to strike America, to attack us, I would have used every resource, every asset, every power of this government to protect the American people."

It is hard to imagine that anyone - even Mr. Bush's fiercest critics - doubts that.

But Mr. Bush's statement illustrates the transition from a political culture where accepting responsibility demonstrated strength to one in which it exposes weaknesses.

Compare the actions of another young president faced with a crisis early in his administration. It was mid-April 1961, and a C.I.A.-organized invasion of Cuba had collapsed at a place called the Bay of Pigs.

"There is an old saying," President John F. Kennedy said, "that victory has a hundred fathers and defeat is an orphan." The president added, "I'm the responsible officer of the government."

Despite the debacle, Mr. Kennedy's popularity increased.

But statesmanship is not always everything it seems, said Michael Beschloss, a presidential historian. Even as Kennedy was taking responsibility, his aides were out quietly - on background as they say in Washington - blaming the fiasco on Dwight D. Eisenhower, who had set the invasion in motion. Eventually, one Kennedy administration official, Stuart L. Udall, blamed Eisenhower in public, which brought a fierce rebuttal from his vice president, Richard M. Nixon, and forced the White House to retreat. President Kennedy, his spokesman, Pierre E. Salinger, said, bears sole responsibility and wanted everyone to know it.

In those days, a leader took responsibility in public and his aides spread the blame only in private. Today, those aides spread the blame on cable TV and only former mid-level officials take responsibility. In the culture of today's politics, presidents may well be afraid to admit they can't make everything perfect.