About the same portion of Americans describe themselves as being liberal (19 per cent) as believe that the world will come to an end in their lifetimes (17 per cent). Right-wingers have so effectively besmirched the term ("wishy-washy liberals", "tax-and-spend liberals", "limousine liberals"), that only a few political martyrs and masochists publicly proclaim their allegiance to the cause once championed by Franklin D Roosevelt. The word preferred by left-of-centre types in the US is "progressive", which hearkens back to the earlier Roosevelt, Teddy, a turbocharged Republican who whipped monopolists and gleefully asserted the power of the federal government.

FDR's robust liberalism focused on social justice at a time when one in four workers had lost their jobs to the Great Depression, and then on social solidarity when the US entered the Second World War. By now, much of that twin legacy has disappeared. But look beneath current political labels and you find a nation still clinging to several liberal ideals. Polls show, for example, that an overwhelming majority of Americans support social security, unemployment insurance and a minimum wage, as well as Medicare for the elderly (courtesy of Lyndon B Johnson), strong environmental protections (Richard Nixon's contribution, surprisingly enough) and a graduated income tax. Most believe that government has no business snooping into people's private lives without cause to believe that they have been involved in crime. The vast majority favour equal civil rights for blacks, women and ethnic minorities. And George W Bush's swagger notwithstanding, most Americans oppose unilateral assertions of US power abroad. An overwhelming majority believe we should work in close concert with our long-standing allies, including France. The shrill, right-wing rantings of radio and television talk-show hosts do not reflect the views of most Americans - or the manner in which they disagree with one another.

The political fault-line in modern America has become cultural. It is about religion, sex and firearms (or, in the vernacular, God, gays and guns). Since 9/11, the culture war has been extended to global terrorism. On the conservative side are Americans who attend church regularly, believe that homosexuality is morally wrong, want the government to ban abortions, take offence at out-of-wedlock births and think they have a God-given right to own any gun they wish. They also want the US to exterminate all terrorists, including anyone with terrorist leanings. Most of the people who think this way reside in rural and southern parts of the nation, towns and small cities, and outlying suburbs. They are the majority in what are now called "red states" - states that lit up bright red on the electronic TV maps late on election day 2000, when returns showed that most of their voters had cast ballots for Bush. They dine nightly on meat, potatoes and a vegetable, watch Fox News, shop at Wal-Mart, and enjoy Nascar races and wrestling on TV. They earn between $20,000 and $60,000 a year - straddling the middle and working classes, doing jobs ranging from mechanic to clerical worker, beautician to physical therapist, and low-level managerial and technical work.

On the liberal side of the cultural divide are those whose church attendance is irregular at best, who harbour far more permissive attitudes toward sex, and think government should control gun ownership and ban handguns and assault rifles. They believe terrorism is a complex problem, requiring better intelligence and more effective ways to win the hearts and minds of Muslims who now opt for suicide missions. They tend to inhabit America's sprawling metropolitan regions in the north-east and on the west coast, the larger cities and the inner suburbs. They are the majority in the "blue states" that went for Al Gore. Their tastes in food tend toward varied national and ethnic cuisines. They watch the major TV networks or public television, and play golf or baseball. They typically earn between $60,000 and $200,000 a year or they earn under $20,000. Cultural liberals tend to be both richer and poorer than cultural conservatives - moderately paid professionals such as teachers, lawyers and social workers, or else low-paid employees such as hospital orderlies, retail and restaurant workers, and hotel personnel. In other words, they are more cosmopolitan than cultural conservatives and more diverse.

Why God, gays and guns? They are proxies for two distinct temperaments that divide the US like a meat axe. On the conservative side is a moral absolutism that views the nation's greatest challenge as holding firm to enduring values in the face of titanic economic and social changes. The common thread uniting strong religious conviction, rigid sexual norms and an insistence on owning a gun is the assertion of authority, typically by men. The task is to apply strict discipline to those who might stray from established norms, and to win what are repeatedly seen as "tests of will". Since 9/11, this has also taken the form of patriotic bravado and stubborn pugnacity. America, say cultural conservatives, must remain the strongest nation on earth. The best way to deal with terrorists is to demonstrate toughness and never waver. Better to be feared than loved; better to be consistent than appear indecisive. The tough-talking, born-again cowboy president, George W Bush, perfectly exemplifies this world-view. Bring 'em on, he says. You're with us or against us.

On the liberal (progressive) side of the cultural divide is a belief in tolerance, reason and law as central tenets of democracy. Americans who hold to this view consider all public issues to be soluble with the correct and relevant information, subjected to objective analysis and full deliberation. Religion and sex fall outside the public sphere because they are inherently private matters. A vibrant democracy must tolerate different beliefs and personal choices. Gun ownership directly affects the public sphere and, as such, is subject to regulation if there are good reasons to limit it. (As there are.) By extension, the battle against global terrorism requires that we be smart rather than merely tough. We have to get our facts straight (Saddam Hussein had no weapons of mass destruction), tell the public the truth (Iraq played no part in 9/11), apply rational analysis (our first priority must be to keep nuclear weapons out of the hands of potential terrorists) and respect international law (work through the UN and Nato, and don't torture prisoners). We also need to get at the causes of terrorism - the hate and hopelessness that fuels it. If you want to understand John Kerry, look no further.

Cultural conservatives condemn liberals as having no strict moral compass, as being "moral relativists" and "flip-floppers". These charges predate the 2004 presidential campaign. Conservatives fear liberals will sell out because they don't know what they stand for. In fact, liberals do have strong beliefs (again: tolerance, reason, democratic debate, the rule of law), but these beliefs seem more about process than substance and do not lend themselves to 30-second soundbites. To liberals, most issues are complicated and nuanced. This attitude drives moral absolutists nuts. American liberals, for their part, worry that the right-wing conservatives are stubborn, intolerant zealots who shoot before they think. Recent history seems to bear out these fears.

Presidential elections in modern America have been about these contrasting world-views since at least 1964. Starting with Barry Goldwater's failed bid in that year and continuing through Nixon, Reagan and the two Bushes, the new right has emphasised moral absolutes and the need for authority and discipline to enforce them. By contrast, Jimmy Carter, Bill Clinton, Gore, and now Kerry have focused their campaigns on tolerance, reason and democracy. Republican candidates repeatedly talk about toughness and resolve, while liberals talk about being correct and thinking problems through. On balance, toughness and resolve have proved the easier sell, especially when American voters are worried about something big.

What about social justice? This part of FDR's liberal legacy has been eclipsed by the culture wars. Odd, when the biggest thing voters worry about is their jobs and pay cheques, and the pay cheque (including wages and job benefits) of most Americans has been declining for two decades, adjusted for inflation. The gulf between rich and poor in America is now wider than at any time since the robber barons of the late 19th century monopolised industry and bribed the government to do nothing about it. Yet, in recent years, Democratic candidates have not dwelt on the subject. They have bought the conventional view that economic populism does not sell because most Americans still want and expect to become rich one day. That is rubbish: upward mobility has just about ground to a halt. And it's circular reasoning. Economic populism would sell if Democratic politicians explained to the public what has been happening and why. To his credit, Kerry hasn't ducked the issue. He has promised to end the Bush tax cuts for people earning more than $200,000 a year and use the proceeds to make healthcare affordable for the working class and the poor.

America is splitting into "two nations" (as John Edwards, the Democratic vice-presidential candidate, has said) because the twin forces of globalisation and technological change are rewarding the educated and well-connected, while punishing the less educated and the disconnected. What to do about this? There are solutions that do not require protectionism and neo-Luddism, solutions much in keeping with the liberal legacy of FDR, but too few of today's liberals have been discussing them and the American public doesn't have a clue. You hear them discussed mostly in the rarefied precincts of university towns such as Cambridge, Massachusetts, and Berkeley, California, whose inhabitants talk to one another and convince themselves that the rest of the nation must be saying the same things. One hopes that the conversation will be much wider if Kerry and Edwards are elected.

Robert B Reich, Hexter Professor of Social and Economic Policy at Brandeis University and visiting professor at the University of California, Berkeley, is the author most recently of Reason: why liberals will win the battle for America (Knopf). He was US secretary of labour under Bill Clinton