Sarah Palin and the Arizona shooting
This week, Republicans in the House of Representatives planned a vote to repeal President Barack Obama's healthcare legislation - the law that first caused the Tea Party movement to erupt on to the national scene in a series of furious encounters with politicians who supported the changes. Now, the vote won't happen. Not this week. The US is no stranger to mass killings, but the shootings in Tucson have prompted a national pause. Instead of trying to repeal healthcare laws, a solemn reflective House will pay tribute to Gabby Giffords, and others hurt or killed in the shootings.
It is too late for this crime to be dismissed as the meaningless act of a madman. It may turn out there was no political motive at all. But the killings have already acquired a meaning. A mood of anxiety about political tone that has existed for months has begun to harden into something more tangible, something that could be a game changer, provoking national soul searching.
After the Oklahoma bombing in 1995, when a right-wing extremist killed 168 people by blowing up a federal building, Bill Clinton used the moment to link the attack with the Republicans' anti-government rhetoric of the time. It worked. Mr Obama may not go that far, but he will stand in silent tribute on the White House lawn later. In a year when he is trying to recapture the centre ground and appear above partisan bickering, it will be a silent rebuke to a certain brand of politics.
Facebook spokeswoman Randi Zuckerberg told ABC News the biggest question on the social-networking site since the shootings has been: "Is Sarah Palin to blame?"
Many will say obviously not.
But some liberals accuse the Tea Party movement of creating a climate of hatred where opponents are seen not as wrongheaded, but as treasonous enemies of America.
Ms Palin bears the brunt of the criticism, partly because of the strength of her language, and partly because her website did carry pictures of what looked like crosshairs of a rifle targeting political constituencies in the US, including that of Gabby Giffords. Ms Palin's aides rather lamely claimed the illustrations were meant to represent surveyors' symbols, leading to one blogger talking about that well-known song: "I surveyed the sheriff." And it is Ms Palin who uses the slogan: "Don't retreat, reload."
On the internet, the right has reacted with fury to these accusations. One website writes: "Despite the fact that Jared Lee Loughner was a psychotic loner with 'left-wing' beliefs according to those who knew him, the establishment has hastily exploited yesterday's tragic shooting in Tucson to demonise conservatives, libertarians and gun owners."
But whatever the bloggers get up to, this week no national figure will indulge in harsh rhetoric, and there will be, for a while, a change in tone.
That is difficult for Ms Palin and her ambitions. As a self-described pit-bull with lipstick, her appeal is in her ferocious attack. But it is hard to believe she will ever again talk of reloading or even targeting opponents. Her trademark bite and bark may ill fit a newly chastened public mood. It may make her less appealing to Republicans, who are already worried that she can't appeal to the centre. Or this moment of concern may pass quickly and it will be back to business as normal before the month is out. Still, I will be listening to her tone very carefully when she makes her next public speech.