"Does the sun ever shine during a Democratic administration?" asked Jon Stewart on a recent edition of The Daily Show, in a nod to the rightwing hysteria over the current White House incumbent. "With Obama in office now, when babies laugh do you hear only the sound of kittens drowning?"

Stewart is a comedian, but this is no laughing matter. The lies perpetrated against Obama – that he is a foreigner, a Marxist, a Nazi, a racist, intent on interning the elderly and euthanising the disabled – and the ensuing hatred and virulence, have reached fever pitch.

I could only rub my eyes in disbelief when I saw footage of one protester standing outside an Obama town hall meeting in New Hampshire with a loaded handgun strapped to his leg, holding a placard proclaiming: "It is time to water the tree of liberty," in reference to the Thomas Jefferson quote "The tree of liberty must be refreshed from time to time with the blood of tyrants and patriots." In Maryland, one man went even further, holding up a sign saying: "Death to Obama" and "Death to Michelle and her two stupid kids".

This militant, rightwing craziness comes on the back of the now-infamous ad published in a Pennsylvania newspaper in May, calling for Obama to be assassinated – and, of course, in the wake of a presidential election campaign in which crowds at Republican rallies shouted "Kill him!", as well as "Treason!" "Terrorist!" and "Off with his head!"

Less than a year on, and under the spurious guise of a "row" over healthcare, we are left, in the words of investigative journalist Chip Berlet, with "a very dangerous situation that can spin off 'lone wolf' individuals who decide now is the time to act against people they see as an enemy."

So why are Republican politicians fanning the flames with talk of "death panels" and Nazism? Why are pundits like Glenn Beck and Rush Limbaugh indulging in what the historian Richard Hoftstader described as the "paranoid style" in American politics? Do they really not understand that this hysterical wave of hatred and fury against Obama goes beyond healthcare reform, or party politicking, or even the country's so-called "culture wars", and now threatens the life of their elected head of state? Do they want to be held responsible, through their extreme and dishonest rhetoric and hate-filled divisiveness, for the death of America's first black president?

Critics might accuse me of being alarmist, not to mention tasteless. To even speculate about the assassination of the president is deemed crass and unseemly. They might point out that this is 2009 and not 1962, and a lone gunman stands little chance against the modern might of the United States Secret Service.

Perhaps. But federal agents are concerned about a rise in the number of heavily armed militia groups springing up around the country. A report this month from the veteran civil rights organisation, the Southern Poverty Law Centre, claimed 50 new extremist militia groups had formed in less than two years, driven by distrust of the government and now angered at having an African-American in the White House. A spokesman cautioned that the groups had not yet reached the point of violent anti-minority, anti-government attacks, "but we seem to be getting there". The report also quoted one senior federal law enforcement official, Bart McEntire, as saying: "You're seeing the bubbling right now. You see people buying into what they're saying. It's primed to grow."

Where will it end? Watching television footage of deranged US conservatives screaming hysterical threats at their elected representatives in town hall meetings across America, some turning up to the protests carrying assault rifles, I can't help but be reminded of similar scenes from Israel in the mid-1990s.

In those days, peace-making premier Yitzhak Rabin could not go out in public without being booed or heckled. He was vilified by the Israeli right in public demonstrations and portrayed as a Nazi and accused of treason by the settler movement. Members of his party were physically harassed and had their lives threatened. Sound familiar?

The mainstream Israeli right was, in the words of Middle East analyst Geoffrey Aronson, "content to lend its aura of respectability to many of these incidents, some of which occurred, without condemnation, during rallies addressed by [Likud] party officials. Netanyahu … saw political advantage in the increasingly poisonous atmosphere."

Rabin, as we know, was eventually assassinated, despite all his security – killed by a lone gunman. But his widow Leah never forgave Netanyahu or the Israeli right for indulging the vitriolic rhetoric of the settlers and fostering the hate-drenched atmosphere that led to her husband's murder.

You might argue that the analogy is flawed and that America is not Israel. Indeed. Israel has lost only one of its leaders to an assassin's bullet. The US has lost four. If, God forbid, Obama were to go the way of Lincoln, Garfield, McKinley and Kennedy, as that Pennsylvania newspaper ad demanded, 21st-century America would be engulfed by violence and protests on a scale not seen since the 19th-century civil war. The country would tear itself apart.

Republicans should beware. They are playing with fire.