Saturday has the potential to be a transformational moment for Barack Obama’s presidency.

Called to deliver the eulogy for his friend and mentor, Edward Kennedy, Obama can — and should — use this moment to reconnect with the values and the ideals that propelled him to the White House.

It will come as a surprise to no one that the president has been asked to deliver this eulogy, as it was Kennedy who inspired, encouraged and ultimately endorsed Obama’s audacious quest for the nation’s highest office.

Obama will well honor the man he referred to on Wednesday to as “a colleague, a counselor, and a friend.” There is no question of that.

The only question is whether Obama will honor the moment and use it, as Kennedy once did, to speak not merely of a life lost but of a cause unbowed.

To do right by Kennedy, Obama must make his words on Saturday more than a eulogy.

He must deliver a renewing address, both for the causes Kennedy championed — of which the first and foremost is universal healthcare — and for the presidency in which the late senator invested as much hope as the most idealistic Obama volunteer.

Obama must speak in the Kennedy tradition when he rises to speak Saturday morning at Boston’s Basilica of Our Lady of Perpetual Help.

To do so, the president will need to begin with an understanding that Ted Kennedy established his reputation not merely as an orator but as the undisputed tribune of contemporary liberalism with his remarkable 1968 eulogy for his slain brother, former New York Senator Robert F. Kennedy.

The 36-year-old senator from Massachusetts, who had until then been seen as little more than a political hanger-on, suddenly shone as the defender of the dream — the champion of the ideals expressed during his brother’s groundbreaking peace-and-justice campaign for the Democratic presidential nomination.

“We loved him as a brother, and as a father, and as a son. From his parents, and from his older brothers and sisters — Joe and Kathleen and Jack — he received an inspiration which he passed on to all of us. He gave us strength in time of trouble, wisdom in time of uncertainty, and sharing in time of happiness. He will always be by our side,” Kennedy began, his voice cracking with emotion. “Love is not an easy feeling to put into words. Nor is loyalty, or trust, or joy. But he was all of these. He loved life completely and he lived it intensely.”

Kennedy then quoted his brother’s words from RFK’s groundbreaking 1966 speech to the young people of South Africa, in which the New York senator declared: “There is discrimination in this world and slavery and slaughter and starvation. Governments repress their people; millions are trapped in poverty while the nation grows rich and wealth is lavished on armaments everywhere. These are differing evils, but they are the common works of man. They reflect the imperfection of human justice, the inadequacy of human compassion, our lack of sensibility towards the suffering of our fellows. But we can perhaps remember — even if only for a time — that those who live with us are our brothers; that they share with us the same short moment of life; that they seek — as we do — nothing but the chance to live out their lives in purpose and happiness, winning what satisfaction and fulfillment they can.”

Ted Kennedy could have rested on his brother’s eloquence.

But that he did not do.

Rather, the young senator from Massachusetts concluded:

My brother need not be idealized, or enlarged in death beyond what he was in life; to be remembered simply as a good and decent man, who saw wrong and tried to right it, saw suffering and tried to heal it, saw war and tried to stop it.

Those of us who loved him and who take him to his rest today, pray that what he was to us and what he wished for others will some day come to pass for all the world.

As he said many times, in many parts of this nation, to those he touched and who sought to touch him: ‘Some men see things as they are and say why. I dream things that never were and say why not.’

With those words, Ted Kennedy grabbed the flag of American liberalism and raised it aloft — reshaping the anguished cry at his brother’s assassination into an inspired call to action.

Obama faces a different task than the one Kennedy performed on June 8, 1968, at St. Patrick’s Cathedral in New York. He is eulogizing a friend, not a brother. He is speaking of a man who died in relative old age, not the youthful victim of an assassination.

And, yet, the stakes are just as high.

Few would seriously debate that Barack Obama has been too cautious, too unfocused in the first months of the presidency that Ted Kennedy imagined would be characterized by: “New hope for justice and fair prosperity for the many, and not just for the few. New hope — and this is the cause of my life — new hope that we will break the old gridlock and guarantee that every American — north, south, east, west, young, old — will have decent, quality healthcare as a fundamental right and not a privilege.”

Obama has an opportunity to honor Kennedy by turning this moment of loss into a moment of renewal.

This is not just what Kennedy would have requested.

It is what the liberal lion would have demanded.

Kennedy close his last speech to a Democratic National Convention — a speech he delivered on behalf of Obama’s candidacy — by roaring: “The work begins anew. The hope rises again. And the dream lives on.”

The president should echo and, more importantly, embrace those sentiments.

If he does, August 29, 2009, could be as transformational a day in Barack Obama’s political career as was June 8, 1968, for Ted Kennedy’s.