There are good explanations for why the use of fear tactics often works politically, and the tactics by which they are often deflected and defused reveals some of the risks all sides bring upon themselves and upon the rest of us when candidates take that path.

One of the most basic functions of government is the protection of its citizens against injury and violence from both foreign and domestic enemies. The need for public reassurance about personal security is thus a longing lodged deep in the reptilian portions of our brains.

It is not surprising, then, that any candidate for the US presidency has to show that he or she can pass the toughness test. Voters need to be reassured that their chief executive has the will, and perhaps even an appetite for, doing whatever it takes to defend the populace.

It is therefore no surprise that the US and nations across the span of human history often turn for leadership to military figures – when military leaders themselves don’t pre-empt that choice - in times of uncertainty or heightened anxiety.

For candidates who do not have strong military credentials, there are a number of other ways they can pass the toughness test. In fact, there are two prongs of the toughness test, and they are often intertwined.

Saber rattling against real or imagined enemies abroad, as well as high-pitched verbal demonstrations of willingness to get tough on domestic criminals and home-grown subversives are both tried and true strategies. Each approach offers candidates a way of harnessing primal fears in service of personal ambition.

Better yet, where candidates can exploit both avenues simultaneously. The promise to protect the public from all enemies domestic and foreign almost always trumps candidates who don’t go the whole distance.

Richard Nixon’s 1972 campaign against George McGovern is the textbook case. He promised to do whatever it took to protect us from the communist menace abroad and to control the civil unrest and protests domestically.

Sometimes, however, a candidate has only one avenue available, and it may serve the candidate well to emphasize strengths with respect to one prong of the toughness test in order to compensate for perceived deficiencies in the other.

Woodrow Wilson was a great example. He is best remembered for his reticence to go to war (though he did ultimately) and his unpopular support for the League of Nations after the war. He was able to sustain his political career without a muscular foreign policy, in large part, because it was coupled with an aggressive effort to mobilize rising anti-immigrant sentiment and fear of radical dissent.

Wilson pushed for the passage of the Espionage Act which gave the government unprecedented authority to suppress speech, prevent public gatherings, and prosecute anyone for critical remarks directed toward the government, its policies, its conduct of the war, and even the flag.

In what became known as the “Palmer raids” his Attorney General A. Mitchell Palmer rounded up more than 16,000 people, held them for lengthy periods without criminal charges, but in the end, few were ever convicted.

The manipulation of ethnic fears provided Wilson with a bonus of political capital that counterbalanced his widely perceived weakness on matters of war and peace.

Recent experience reveals a similar, though far less extreme example. During his 1992 New Hampshire primary Governor Bill Clinton suspended his campaign schedule when he went back to Arkansas to preside over the execution of Rickey Ray Rector.

Rector, as anti-death penalty activists and intellectual disabilities advocates recall vividly, was an African-American inmate whose mental retardation was so severe that he asked the staff on death row to put the dessert from his last meal aside so that he could have it after his execution.

Governor Clinton, at the time, was facing a barrage of criticisms over his responses to inquiries about the way he avoided military service. Faring well on the foreign policy part of the toughness test was not an option, but, whatever his intentions, the trip reinforced the view that Clinton was a new Democrat who was willing to do the tough-minded, law and order business that might be required of him.

For a brief moment it looked as if the politics of fear might not be the winning strategy in 2008. Rudy Giuliani , after all, was the consummate 9/11 candidate, and he exited early. But now we have an appeal to fear emerging within the Democratic primary.

Many have wondered aloud whether Senator Clinton’s use of the foreign policy fear gambit may not come back to haunt the Democrats.

If Obama is the nominee, Clinton’s words will surely make their way into numerous Republican television ads and speeches. If Clinton is the nominee, McCain will see her bid and raise the stakes well beyond what she can likely match.

McCain will be positioned to pursue a more robust campaign based on fear. For an all too brief moment, McCain opposed President Bush on matters of permitting torture and curbing domestic civil liberties. But his dramatic legislative turn-around leaves him free, should he choose, to go beyond anything that any Democrat is likely to find the stomach for.

In fact, it is difficult to find the appropriate stopping point once fear becomes the central case for one’s campaign. Once the shared premise of both campaigns is that our most vital concern is the dangerous world we inhabit, the candidate who can come up with the most threats and the most enemies almost always wins. The rest of us almost always lose.