"We must love one another or die." After 9/11, W.H. Auden's poem "September 1, 1939" crackled through the Internet because that line spoke to the urgent longing many Americans felt. I cited it myself in a column three years ago this week. Auden may have repented of that sentiment, once Hitler's purpose became clear, but the words had special poignancy in 2001. A tremendous national cohesion bound Americans, and citizens of the world sent us messages of concern and sympathy. Love emerged as a public virtue. The catastrophe yielded a rare communion.

How long ago that seems. Has America ever felt so divided within itself, ever been so cut off from other nations? As I heard a friend observe the other day, the election is laying bare a split between two groups, each of which views the other with similar mystified alarm: How can they think that way? Don't they realize what disasters will follow if their candidate wins? Partisan disdain is as absolute as it is mutual. Both sides recognize the stakes of the election as mortal, with one fearing the flames of unnecessary wars, and the other fearing yet more attacks like 9/11. Indeed, fear may be the only thing the blue states and the red have in common, but what we mainly seem to fear is the other side.

An adaptation of the Auden line was used to express a public fear once before, during an election that not only exposed an early form of this national division, but made it worse. The infamous "daisy" commercial, in which a young girl is seen counting the petals of a flower, as a voice counts down to a mushroom cloud explosion, ended with Lyndon Johnson himself saying, "We must either love each other. Or we must die."

The commercial branded Barry Goldwater as a nuclear madman, effectively suggesting that a vote for him was a vote for death, and so the ad was not really about love at all. The rage that conservatives felt at the smearing of their candidate (despite Goldwater's expressed willingness to entertain the use of nukes) was part of what mobilized their eventual arrival as the dominant force in American politics. What evolved into the unyielding red-blue divide of the current electoral season can be understood as having begun in 1964.

The daisy commercial resonates still because a subliminal fear of nuclear war planted itself in the American unconscious. Nuclear fear is unlike what might be called normal fear, because the threat transcends concern for individual mortality, calling into question the mortality of the human species -- the fate of the Earth, in Jonathan Schell's phrase. The death of the future itself is what we fear. The taboo broken by Lyndon Johnson's daisy commercial assault on Goldwater was the taboo against speaking openly of nuclear war as a present danger. After 1964, the danger did not go away, but we Americans would never openly contemplate it again.

Ironically, because our nuclear fear was never fully reckoned with, we did not remove its source -- the massive nuclear arsenal -- even after its Cold War justification vanished. And today's endemic American fear of terrorism, whether defined in red politics or blue, is rooted in this unacknowledged fear of nuclear catastrophe. After decades of implicitly waiting for the mushroom cloud to appear over the nation, we saw the clouds of ash rising from the World Trade Center as a version of that horror. As I heard the scholar John Dower observe, the use of the term "Ground Zero" in New York is an unconscious appropriation of the authentic Ground Zeros in Japan. That is why 9/11 traumatized us out of all proportion to the scale of destruction, which, while tragic, was hardly world-historic.

America is palpably afraid, but of what? The appeal in Auden's line is rooted in the sense that something new has happened in history. But the change to which we are desperately looking for ways to respond occurred not in 1939, and not in 2001, but in 1945, when we ourselves brought forth the mushroom cloud that haunts us still.

To say, "We must love one another or die" is only to say that, among nations, a new communal politics defines the only future there can be. But such life-or-death global change must begin here, and within this nation, politics is more contemptuous than ever. If hate defines us at home, how can we hope for love abroad? There is the thing to fear.