A few weeks back, Glenn Beck marshalled the forces of the far right to rally in Washington, DC. It was a Tea Party extravaganza, which, ludicrously, claimed to be about resurrecting the role of faith in American life, rather than promoting a partisan political agenda. In an act of startling chutzpah, Beck, a Fox TV cable show host who has morphed into one of America's most effective, and therefore dangerous, conservative demagogues, managed to invoke the civil rights legacy of Martin Luther King.

Now, in reaction to Beck's triumph, the satirists Jon Stewart and Stephen Colbert have both announced their own, competing, marches on DC. Stewart's putative event is titled "Rally to Restore Sanity". Colbert's is deliciously labelled "March to Keep Fear Alive". Unlike Beck's rally, these are not serious political events, with serious, large-scale grassroots movements behind them; rather, they are intended as satirical commentaries on a debased political culture.

In Beck's case, the protest was not to pushfor anti-poverty programmes, or social justice policies, or serious attempts to end ongoing racial disparities in the United States, but for an anti-government, anti-incumbents, anti-tax message numbing both in its juvenile language and its paltry understanding of the complexities of modern-day America. Deep intolerance bedecked with the magnificent language and ideals of King; it ranks as one of the most Orwellian moments in contemporary politics.

Beck claims to be something of a historian and scholar; certainly, he's a whole lot better-read and probably smarter than are many of his followers. But, as the actress Jamie Lee Curtis points out so witheringly, in the movie A Fish Called Wanda, to her incredibly stupid sidekick, played by Kevin Kline, monkeys can peruse philosophy – "they just can't understand it". Beck is a small man, a bully, a narcissist besotted with his ability to sway the crowd. He appeals not to reason but to fear, not to careful historical interpretations of world events but to a smorgasbord of conspiracy beliefs and rumours fuelled by disinformation. He is, in many ways, the Father Coughlin – a brilliant, but destructive, fascist radio broadcaster during the Great Depression – of our age.

Stewart and Colbert's shows are, correspondingly, perfect reflections of our era: their audiences are smart, sassy, liberal, multicultural and, above all, perturbed by the sorts of cultural and political trends that turn people like Glenn Beck and Sarah Palin into superstars. But, for those same reasons, they are limited. Stewart and Colbert can marshal their audiences to laugh; they can highlight absurdities and hypocrisies; and they can win entertainment awards for their efforts. But they can't mobilise the masses.

Beck, that's another story. He can move millions of Americans to fury with his demagoguery; he can stoke rage, a sense of victimhood, a chronic sentiment of grievance and foul-play.

A few years back, I started realising just how strange American politics was becoming when I would pick up a copy of the satirical New York paper the Onion and find myself thinking the bizarr-o headlines were actually as plausible as the real life news stories one could read in the serious press. Since then, that weird down-the-rabbit-hole sensation has gotten more pervasive.

This week, we learned that Christine O'Donnell, the newly nominated Republican Party Senate candidate in Delaware believes that masturbation is a form of adultery and that evolution is "soft science". News organisations report that about a fifth of the American electorate believes that President Obama is a Muslim. And last week, neoconservative sage Dinesh D'Souza wrote in Fortune magazine opined that it was Obama's Kenyan tribal mindset that explained his political travails and supposed "anti-business" fanaticism. This is, truly, the Season of the Onion.

So, the Stewart-Colbert initiatives March to Keep Fear Alive and the Rally to Restore Sanity are entertaining, diversionary skits during the Season of The Onion. But the Tea Party's rallies and the politics of pure, unfiltered rage seen on display in this current election season are far more significant, and ultimately far more subversive of the fabric of the American body-politic.

Get too close to the onion, and, eventually, it makes you cry.