Forty years ago today, on the Democratic presidential campaign trail in Indiana, one of the most remarkable moments in American political history occurred.

New York Senator Robert F. Kennedy was locked in a race for the Democratic nomination with another liberal insurgent, Minnesota Senator Eugene McCarthy, and the party establishment’s emerging choice: Vice President Hubert Humphrey.

Kennedy had not yet won any primaries. His candidacy remained untested, as did his faith that it was possible to cross lines of racial division and unite a nation that was struggling to overcome the awful legacies of racial segregation, discrimination and fear.

Kennedy arrived on April 4, 1968 to make his stand in Indiana, a racially-divided state where Alabama segregationist George Wallace had run well in a Democratic presidential primary four years earlier.

That night, before he appeared in Indianapolis, Kennedy learned that the Rev. Martin Luther King Jr., the Nobel Peace Prize-winning champion of the nation’s civil rights movement, had been assassinated in Memphis.

Kennedy was scheduled to speak to a large outdoor rally. The Indianapolis police said they could not assure his safety. His own staff counseled that the event should be canceled.

But Kennedy chose to appear.

Here is what the white senator said to his overwhelmingly African-American audience:

I have some very sad news for all of you, and I think sad news for all of our fellow citizens, and people who love peace all over the world, and that is that Martin Luther King was shot and was killed tonight in Memphis, Tennessee.

Martin Luther King dedicated his life to love and to justice between fellow human beings. He died in the cause of that effort. In this difficult day, in this difficult time for the United States, it’s perhaps well to ask what kind of a nation we are and what direction we want to move in.

For those of you who are black – considering the evidence evidently is that there were white people who were responsible – you can be filled with bitterness, and with hatred, and a desire for revenge.

We can move in that direction as a country, in greater polarization – black people amongst blacks, and white amongst whites, filled with hatred toward one another. Or we can make an effort, as Martin Luther King did, to understand and to comprehend, and replace that violence, that stain of bloodshed that has spread across our land, with an effort to understand, compassion and love.

For those of you who are black and are tempted to be filled with hatred and mistrust of the injustice of such an act, against all white people, I would only say that I can also feel in my own heart the same kind of feeling. I had a member of my family killed, but he was killed by a white man.

But we have to make an effort in the United States, we have to make an effort to understand, to get beyond these rather difficult times.

My favorite poet was Aeschylus. He once wrote: “Even in our sleep, pain which cannot forget falls drop by drop upon the heart, until, in our own despair, against our will, comes wisdom through the awful grace of God.”

What we need in the United States is not division; what we need in the United States is not hatred; what we need in the United States is not violence and lawlessness, but is love and wisdom, and compassion toward one another, and a feeling of justice toward those who still suffer within our country, whether they be white or whether they be black.

So I ask you tonight to return home, to say a prayer for the family of Martin Luther King, yeah that’s true, but more importantly to say a prayer for our own country, which all of us love – a prayer for understanding and that compassion of which I spoke. We can do well in this country. We will have difficult times. We’ve had difficult times in the past. And we will have difficult times in the future. It is not the end of violence; it is not the end of lawlessness; and it’s not the end of disorder.

But the vast majority of white people and the vast majority of black people in this country want to live together, want to improve the quality of our life, and want justice for all human beings that abide in our land.

Let us dedicate ourselves to what the Greeks wrote so many years ago: to tame the savageness of man and make gentle the life of this world.

Let us dedicate ourselves to that, and say a prayer for our country and for our people. Thank you very much.

One month later, when Indiana Democrats voted, they gave Kennedy a resounding victory — he won 42 percent of the vote to uncommitted favorite son for Vice-President Hubert H. Humphrey, who entered the campaign too late to meet the filing date to enter the Indiana primary; he eventually came in second in the spirited three-way race in the Indiana primary (Kennedy drew 42 percent to 31 percent for the state’s governor, who was running as a stand-in for Humphrey, and 27 percent for McCarthy).

One month after the Indiana primary, Kennedy was himself assassinated in Los Angeles. The shots that felled the senator stilled not merely a political campaign but a hope that the election of 1968 might somehow deliver America from the trauma of the King killing and all the other painful challenges facing a country that had not then — and has not yet — realized the dream of reconciliation and renewal.

But Kennedy’s words, so powerful and so relevant to our current circumstance, keep that hope alive — and with it the faith that a truer and better politics might yet deliver us from the trauma of 1968 — and of 2008.