When conservatives turn to fury, liberals take solace in irony. And so it has come to pass that, in this political season of rightwing rage and thunder, the big, no-holds-barred liberal rally on this upcoming final weekend of the campaign season will be hosted not by Barack Obama or Bill Clinton or any other leading Democrat party politician, but the television host Jon Stewart.

The "Rally to Restore Sanity" will be held on Saturday on the National Mall in Washington – the hallowed greensward covering the 15 blocks from the Capitol building to the Washington Monument. Expectations are for vast crowds, the better to prove that the fabled "enthusiasm gap" of this election between liberals and conservatives has closed. Into the bargain, Stewart's Comedy Central doppelganger, Stephen Colbert, staying firmly in his character of plastic rightwing talkshow host, will sponsor a "March to Keep Fear Alive" the same day, also in Washington, appealing to the same world-weary demographic.

I appreciate Stewart and Colbert. The Democrats, by and large a stable of geldings, have no purchase on how to inform America about how extreme and loony the other party has become. When some fresh piece of madness is exploding across our landscape – Christine O'Donnell and witchcraft, the brain-crushing demagoguery in August about the proposed mosque in Manhattan – it needs someone like Stewart or Colbert to step in and introduce perspective, and to expose what's happening for the nuttiness it is, while Democrats simper in the corner in fear of Fox News. And in the absence – no, make that the utter absence of any coherent Democratic theme for this election, Stewart in particular has taken it on himself to try to provide one. And good for him.

But potential danger lurks here, because Stewart is trying to pitch his event to an audience that I don't think is really his. From the time he announced the rally in mid-September, Stewart has pegged it as being aimed at the sensible, busy Americans who simply want a non-ideological sanity to prevail in Washington. He spoke of the "seventy to eighty percenters" – the people in the great middle – who eschew both extremes. The night of the announcement he mocked both the rightwing Tea Party movement and the liberal antiwar group CodePink, whose members interrupt congressional hearings with various boisterous antics while dressed like drag-queen versions of Jackie Onassis. Playing off a phrase known instantly in America and dating back to Louis Farrakhan's 1995 Million Man March, Stewart wants a "Million Moderate March". To get his viewers into the intended spirit he offered some samples of the sort of placards he'd like to see at his rally. In this age, when Tea Partiers march carrying placards of Obama wearing a keffiyeh or sporting a Hitler moustache, people know they should pay particular attention to placards; Stewart suggested that an emblematic one for his event would read: "I disagree with you, but I'm pretty sure you're not Hitler."

The conflict arises in the fact that this sober and earnest middle is not really Stewart's audience. Stewart's core audience is news-junkie liberals. As is Colbert's. It's people like National Public Radio host Terry Gross, who, in a recent live dialogue at Manhattan's venerable 92nd Street Y, thanked Stewart for being the last thing she sees at night, which permits her to "go to bed with a sense that there is sanity someplace in the world". It's young urbanites and students. It's the out-of-place blue fish swimming the waters of the vast, red, middle-American sea. The moderate married couple with a child or two who are too busy for politics – his ideal marcher – are for the most part probably also too busy for Stewart.

This points up one problem with the Stewart approach that liberals don't talk about much, which is his occasional and to me very awkward attempt to make Republicans laugh too. I used to watch the show more devotedly in the Bush years, and I thought I began to notice that Monday nights (the Daily Show runs Monday to Thursday) were make-fun-of-Democrats nights. His audience tried gamely to laugh at routines about John Kerry, but they wanted Dick Cheney jokes. Stewart evidently felt (and still feels) the need to have something vaguely resembling balance. Well, it's a noble impulse. But it always felt to me like he was straining for a neutrality that wasn't there in his heart. He seems to be pitching the rally toward that same notion of neutrality. But I doubt that's what will show up on Saturday, and that's what worries me.

Now, digress with me for a moment to the memorial service for Paul Wellstone, the crusading, proud-to-be-liberal Minnesota senator killed tragically in an plane crash while en route to a friend's funeral, a week or so before the 2002 election. The service, held in a large arena just days before the voting, was 90% positive and sincere. But one speaker inappropriately chided Republican senators who were present, and some stupid attendees booed when images of those senators appeared on the screen.

The rightwing media seized on those incidents and over the next few days turned what was mostly a fine event into a Nurembergesque hatefest. Al Franken, then a commentator and now a senator who holds Wellstone's seat, thoroughly debunked the lies told about that rally – but after the fact: at election time the conservative commentariat made sure their version of events dominated the media coverage of the last few days of the campaign.

So let's go back to this question of signs. I can guarantee you that many of the clever liberals planning on attending the rally are dreaming up very clever signs. You can even Google "Jon Stewart rally signs" and fetch 256,000 results. I can further guarantee you that Fox News and its fans will have many cameras on the rally in Washington, on constant lookout for posters that demonstrate intolerance, elitism and so on. And Fox, and Rush Limbaugh and the rest will do everything they can to turn the rally into another liberal hatefest.

I trust Stewart is aware of this and already planning his response. But unlike Stewart, Fox News doesn't reserve Monday nights for equal time. I don't doubt that the rally itself will be excellent in real life. It's what it can be made to look like afterwards that worries me.