The preview for Al Gore’s An Inconvenient Truth declared it “by far the most terrifying film you will ever see. It will shake you to your core.” The movie never shied from summoning the prospect of global doom–and summoning it again and again. Yet the movie didn’t leave its audiences quivering in their seats, paralyzed by the thought of looming catastrophe driven by powerful forces far greater than any individual. Along with increasingly harrowing communiqués from the UN’s Intergovernmental Panel on Climate Change, and natural disasters like Hurricanes Katrina and Rita, and now huge fires across the West, the film spurred a planet to action. Individuals and governments alike are doing things that just two years ago seemed politically unattainable eco-fantasies.

But just as fear is starting to pay dividends for the planet, many liberals are turning their backs on this most powerful of political emotions. An idea that seems to keep bubbling up in Democratic and progressive circles is that everyone from environmentalists to healthcare advocates to Democratic candidates should keep their messages upbeat and hopeful.

It’s an appealing conceit for the confrontation-averse: if fear doesn’t work, we can just go on doing the less emotionally taxing and less controversial work of delivering happy little sermons. For that reason, just three years after running a cheerful and largely fearfree 2004 convention that was blown away by the Republicans’ rhetorical mushroom clouds, lots of liberals are latching onto some recent psychological studies that suggest anxiety has an expiration date.

They say that President Bush’s use of fear is generating diminishing returns: each time Bush tries to evoke fear, it works less as more people clue into his deceptions.

“Barring another assault on American soil, the moment of September 11–and the reminder of mortality that it brought–may well have passed,” wrote John Judis in a recent article in The New Republic. “And with it, too, the ascendancy of politicians who exploited the fear of death that lies within us all.”

Judis was interpreting a series of studies by psychologists Tom Pyszczynski, Sheldon Solomon and Jeff Greenberg that found clear evidence that evoking people’s fear of death made them more defensive of their world view, more hostile to foreigners, more willing to lash out violently at people of different political or religious beliefs and more drawn to charismatic leaders.

But these studies also found that it’s possible to limit or reverse these effects. Urging people to make careful decisions rather than rash ones, priming people to think about the value of tolerance or, critically, alerting them to the possibility of manipulation, limits the ability of death to generate a defensive response. The antifearmongers have embraced this conclusion, arguing that Republicans’ use of fear in the past fourteen years has exhausted its ability to generate the intended response and that it may even be producing a backlash.

So while it’s true that the impact of politically motivated Homeland Security alerts and strategically timed Al Qaeda arrests may be ebbing, that doesn’t mean dread is dead.

Indeed, President Bush’s monomaniacal focus on terrorism and Iraq has effectively distracted people from other lethal threats: disease, crime, state-sponsored nuclear attack and the destruction of the natural world. Clever politicians who remind people of those threats will continue to leverage the immense political power of the fear of death.

Leading Republicans know this and are desperately seeking something new that can trigger the fear of death in a way that turns people to the GOP. Some have turned to the prospect of a nuclear-armed Iran to trigger people’s defense mechanism. But more savvy Republicans recognize that the WMD threat, even if real this time, may not resonate with a war-weary and cynical public. That helps explain the recent GOP tendency to cast immigrants as murderers, gang members and lepers. If you can no longer get Americans to believe that a distant Middle Eastern country presents an existential threat, you might be able to get the same reaction by freaking them out about the gardener.

Many Democrats, on the other hand, have been reluctant to appeal to fear –either because they think doing so is unethical, because they think fear doesn’t work for Democrats or, most commonly, because they lack the gumption to engage Americans’ emotions and not just their minds.

Foremost among the skeptics of using fear for progressive politics is Alex Gourevitch, who wrote an article in the journal n+1 criticizing environmentalism for being a “politics of fear” oriented around “threats to the conditions of life” rather than “social controversies over power and distribution.”

“Environmentalism is not just some politics,” Gourevitch was quotedas saying in the New York Times‘s City Room blog. “It’s built on the idea [of] a collective threat that makes security the basic principle of politics and makes the struggle for survival the basic and central aim of our social and political life. This, to me, is not a progressive politics at all.”

Progressives, environmentalists and Democrats who indulge these reservations are forgetting the vital role fear has played in American politics from the beginnings of the nation–and how the use of that emotion has been indispensable to achieving progressive change and to electing Democrats.

Indeed, the founding fathers were rarely shy about summoning the fear of death to achieve their political aims. Although we tend to remember the American Revolution as one fought primarily over economic and legal grievances, its authors knew that nothing short of primal fear would inspire vast numbers of their countrymen to support a rupture with Great Britain and risk life and liberty in war. In the Declaration of Independence’s famous catalog of grievances against King George III, Thomas Jefferson saves his most powerful indictment for last: the king had “endeavoured to bring on the inhabitants of our frontiers, the merciless Indian Savages whose known rule of warfare is an undistinguished destruction of all ages, sexes and conditions,” a claim that tapped into the most primal of fears for the white colonists clinging precariously to a fragile existence in the midst of a vast wilderness. It was Lou Dobbs for the Enlightenment.

In the 1960s, John F. Kennedy evoked the terror of nuclear holocaust to beat Nixon, Lyndon Johnson used the threat of black violence to corral support for his civil rights bill, and in 1992 and 1996 Bill Clinton skillfully played on worries that Republicans would cut Medicare and Social Security to trigger the fear of dying from lack of resources to pay for healthcare.

These politicians knew instinctively that politics is at least as much about scaring voters about the other candidate as presenting a positive agenda. And it’s an insight that has lately been validated by political science and psychological research.

In his 2006 book, Campaigning for Hearts and Minds, political scientist Ted Brader found that fear in advertising worked far better than positive emotions like hope or enthusiasm. His research showed that fear is especially potent in prompting people to re-evaluate their beliefs.

That’s because when people are afraid, they stop relying on their preconceptions. A near-crash, for example, causes us to rethink how fast to go; a study about the dangers of pesticides makes us re-evaluate what we eat; the mugging of a neighbor leads us to choose a safer route home.

In politics, fear produces the same result. Brader found that watching a single political ad that appealed to fear, buried within a thirty-minute newscast, produced an eleven-point jump in support of the candidate who ran the ad compared to a control group. It also had the more generally positive civic effect of prompting people to seek additional, independent sources of information about the candidates–undermining more traditional views that negativity in politics suppresses interest and dumbs down the debate.

In contrast, Brader found that positive emotional ads aren’t persuasive at all. Instead, they reinforce existing beliefs. People who already support a candidate become even more supportive after seeing an enthusiasm-inducing ad, making that person more likely to actually get out and vote for the candidate or volunteer in the campaign. That makes them useful for candidates who are ahead in the polls or who are campaigning in areas where they already have majority support. The most famous hopeful ad in history, Ronald Reagan’s 1984 “Morning in America,” worked so well because Reagan was already far ahead in the polls when it ran; its purpose and effect was not so much to persuade Mondale voters or undecideds as to reinforce existing support for Reagan and make sure people actually got out to the polls for him.

Fear isn’t just a good persuader; it’s also good at conquering apathy, arguably the progressive movement’s greatest enemy. To quote Samuel Johnson, “When a man knows he is to be hanged in a fortnight, it concentrates his mind wonderfully.”

As powerful as fear is, however, it’s nowhere near as powerful as the combination of fear and hope. While Lyndon Johnson evoked the fear of death more powerfully and more consistently than his opponent Barry Goldwater, he combined that message with one of security, warmth and hope. We remember his ” Daisy” ad for its blinding mushroom cloud but forget the message Lyndon Johnson recorded over it: “We must love one another or die.”

But evoking fear first is essential to tuning people into a message of hope. The politicians we remember as fonts of hope, Lincoln and FDR in particular, were those who had no need to paint scary images with their language–fear had already gripped the land. Even FDR used fear when he was trying to build popular and Congressional support for aid to America’s democratic allies before the attack on Pearl Harbor brought America officially into the war.

FDR, Jefferson and other successful politicians knew what some of today’s cautious Democrats don’t: there are things out there that are legitimately scary. People look to leaders to identify those threats and rally the nation to tackle them–and not cede the use of this most powerful of emotions to those who would exploit it for evil.