Edward M. Kennedy says he is running for president to "preserve the old dream against new dangers" and speaks of taking "the course compelled by events." That does not quite explain it. What unfolded in Faneuil Hall today, as Kennedy stood beneath the oil canvases of Washington and Webster and the marble busts of the Adamses, represents the beginning to the last act in the longest, most painful political drama in American history.

The crowd of Kennedy friends and political loyalists who packed the narrow balconies and front rows of the old hall cheered as the widows of his brothers arrived. They cheered even louder as his mother, Rose, took her place among the family. Everyone there knew they were applauding as much the political past as the present. Nineteen years after his brother Jack, and 11 years after his brother Bob began their presidential campaigns, the last of the Kennedy brothers finally had entered the lists.

That this day would arrive hardly is surprising. For nearly a generation, American politics has revolved around the Kennedy story of successed and tragedies. Those graves in Arlington are testimony to the political changes wrought by the deaths of the brothers.

The assassination in Dallas made Lyndon Johnson president. The one in Los Angeles quite likely put Richard Nixon in the White House. The division within the Democratic Party that followed those murders opened the way for the only true political outsider of this century, Jimmy Carter, to become president.

The question today is not why Ted Kennedy runs for president, but why in this year when all political logic dictates otherwise.

His brothers may have earned the reputation, perhaps inaccurately, for being political risk-takers, but Ted Kennedy has been more circumspect.He was opposed, for instance, to his brother Bob's race in 1968 and had shunned every appeal to become a candidate in all presidential contests since.

Besides, 1984 was far more promising politically -- the personal questions surrounding the fatal accident at Chappaquiddick a decade ago and his relationship with his wife might have receded further in the public memory. And to challenge a weakened Democratic president in a time of trouble obviously creates damaging political divisions and possibly lasting biterness. He stood far more to gain by supporting Carter and waiting for his turn to come.

But as long as a year ago, in the summer, he was musing aloud to a political confidant about the possibility of how he could "take" the president. In the last 12 months he clearly was carefully watching Carter's changing political fortunes.

As the president's troubles worsened and his popularity plummeted, Kennedy was urged privately by many leading Democrats in Congress to consider making the race. They were concerned that the weakened Carter presidency could result in general defeat next November and the loss of Congress.

By late last spring, when gasoline lines lengthened and a political fire-storm broke out after Carter moved to decontrol the price of oil, Kennedy was saying privately to at least one important Democrat that he would assess his political situation in the coming summer months: it no longer might be a question if he would run, but when, he said.

Some of his political conselors, who served his brothers in other days, insist that none of them would ever have urged Kennedy to run. The Prospect prospect of another assassination was too compelling and too real. If Kennedy were to become a candidate, it would have to be his own decision. That, these people say, is what happened.

The senator had convened a number of small meetings at his home over many months.The first series explored, with key figures from his Senate staff and a handful of older Kennedy intimates, the politics of national health insurance: how to implement it, how to focus national attention on it, how to address the media about it.

Fred Dutton, a counselor to all three brothers, remembers thinking to himself when he attended the first of these meetings, "I wonder if this is health insurance or something else?"

The same group explored questions about the state of the nation politically and where America is heading in the 1980s as the dedication of the John F. Kennedy Library in Boston approached last month. But none of these gets to an underlying fact that probably was more decisive in Kennedy's decision -- the personal relationship between Carter and Kennedy.

Whether Carter could have gained a powerful political ally by courting Kennedy and soliciting his help probably cannot be answered. The fact is, he didn't. Their relationship, although always correct, grew more distant as the president's difficulties increased.

Kennedy knew, too, that some important members of the Carter White House spoke disparagingly of him in private, and some of those remarks included references to the senator's personal life style and raised the kinds of questions about personal incidents and political values that Kennedy has faced continually in recent days. Kennedy was, as one key Carter White House aide said privately, "A left-wing demagogue."

The president seemed almost to welcome the fight, with such public remarks about his ability to "whip his ass" if Kennedy chose to run.

These situations, coupled with his own reflections about American leadership in the 1980s and hearkening back to his brother Jack's calls for new frontiers in the 1960s, finally brought him to Fanueil Hall today.

During his speech today, Kennedy referred to these complex emotions, motives, and rationales for running.

"for many months, we have been sinking into crisis," he said. "Yet, we hear no clear summons from the center of power. Aims are not set. The means of realizing them are neglected. Conflicts in direction confuse our purpose. Government falters. Fear spreads that our leaders have resigned themselves to retreat. This country is not prepared to sound retreat. It is ready to advance. It is willing to make a stand. And so am I."

He had, he said, no illusions about seeking the presidency. If he harbors any, they must have been dissipated by the continuing barrage of questions he keeps facing about his character.

In recent days he has spoken of reliving the scene at Chappaquiddick a thousand times, and remarked, "there isn't a day of my life that goes by in which I don't feel a sense of anguish and a sense of loss about it." But still they keep coming.

His answers obviously satisified the partisans from the past who packed Faneuil Hall today. They cheered him, and hissed when a reporter asked questions about his marital situation. As they left that place where American patriots plotted their revolution centuries before, they told stories of other campaigns and othe Kennedy races run -- and won.

As Kennedy made his way to the throngs waiting outside, those inside the hall could hear his voice calling out to his first crowd as a presidental candidate. In tones that evoked memories of 1960, he spoke about carrying the campaign "to the lenght and breadth of the nation."

Whether hisss words and his style will stir emotions and win support of Americans of the 1980s embraces the question that will form the final chapter of this last Kennedy and his campaign.