WASHINGTON – Sad yet riveting, like a wreck by the side of the road, Cindy Sheehan, a plaything of her own sincerities and other peoples’ opportunisms, has already been largely erased from the national memory by new waves of media fickleness in the service of the public’s summer ennui.

But before she becomes fully relegated to the role of opening act for more durable luminaries at anti-war rallies, prudent Democrats – those political snail darters, the emblematic endangered species of American politics – should consider the possibility that, although she was a burr under the president’s saddle for several weeks, she is symptomatic of something that in 2008 could cause the Democratic Party a sixth loss in eight presidential elections. That something is a shrillness unlike anything heard, in living memory, from a major tendency within a major party.

Many warmhearted and mildly-attentive Americans say the president should have invited Sheehan to his kitchen table in Crawford for a cup of coffee and a serving of that low-calorie staple of democratic sentimentality – “dialogue.” Well.

Since her first meeting with the president, she has called him a “lying bastard,” “filth spewer,” “evil maniac,” “fuehrer” and the world’s “biggest terrorist” who is committing “blatant genocide” and “waging a nuclear war” in Iraq. Even leaving aside her not entirely persuasive contention that someone else concocted the obviously anti-Israel and inferentially anti-Semitic elements of one of her recent e-mails – elements of a sort nowadays often found woven into ferocious left-wing rhetoric – it is difficult to imagine how the dialogue would get going.

He: “Cream and sugar?”

She: “Yes, please, filth spewer.”

Do Democrats really want to embrace her variation of the Michael Moore and “Fahrenheit 9/11” school of political discourse? Evidently, yes, judging by the attendance of 12 Democratic senators at that movie’s Washington premiere in June 2004, and by the lionizing of Moore at the Democratic Convention – the ovation, the seating of him with Jimmy Carter.

If liberals think that such flirtations with fanaticism had nothing to do with their 2004 defeat, they probably have nothing to learn from what conservatives did four decades earlier. But for the record:

In the 1960s, just as conservatism was beginning to grow from a fringe tendency into what it has become – the nation’s most potent persuasion – it was threatened by a boarding party of people not much, if any, loonier than Sheehan. The John Birch Society, whose catechism included the novel tenet that Dwight Eisenhower was an agent of the Kremlin, was not numerous – its membership probably never numbered more than 100,000 – but its power to taint all of conservatism was huge, particularly given the media’s eagerness to abet the tainting. Responsible conservatives, especially William F. Buckley and his National Review, repelled the boarders, driving them into the dark cave where, today, they ferociously guard the secret of their size from a nation no longer curious about it.

MoveOn.org, which claims 3.3 million members and is becoming a tone-setting tail that wags the Democratic Party dog that is mostly such tails, adopted Sheehan during her Crawford demonstration, organizing 1,627 vigils around the country to express solidarity with her. But the Democratic Party, whose democratically elected chairman is Howard (“I Hate the Republicans and Everything They Stand For”) Dean, is not ripe for lessons in temperate rhetoric, which may be why the Republican Party has far fewer worries than it deserves.

If Hillary Clinton has half the political sense her enthusiasts ascribe to her, she must be deeply anxious lest all her ongoing attempts to adopt moderation as her brand will be nullified by the increasing inclination of her party’s base to succumb to siren songs sung by the likes of Sheehan. But, then, a rapidly growing portion of the base is not just succumbing to those songs, it is singing them.

George Will is a Washington Post columnist. Contact him by writing to georgewill@washpost.com.