We all know what punishment fits the crime of murder, or even potential murder, if the security of post-9/11 America is at stake. We all know how self-appointed “patriotic” martyrs always justify taking the law into their own hands.

Obama can hardly be held accountable for Ayers’s behavior 40 years ago, but at least McCain and Palin can try to take some responsibility for the behavior of their own supporters in 2008. What’s troubling here is not only the candidates’ loose inflammatory talk but also their refusal to step in promptly and strongly when someone responds to it with bloodthirsty threats in a crowded arena. Joe Biden had it exactly right when he expressed concern last week that “a leading American politician who might be vice president of the United States would not just stop midsentence and turn and condemn that.” To stay silent is to pour gas on the fires.

It wasn’t always thus with McCain. In February he loudly disassociated himself from a speaker who brayed “Barack Hussein Obama” when introducing him at a rally in Ohio. Now McCain either backpedals with tardy, pro forma expressions of respect for his opponent or lets second-tier campaign underlings release boilerplate disavowals after ugly incidents like the chilling Jim Crow-era flashback last week when a Florida sheriff ranted about “Barack Hussein Obama” at a Palin rally while in full uniform.

From the start, there have always been two separate but equal questions about race in this election. Is there still enough racism in America to prevent a black man from being elected president no matter what? And, will Republicans play the race card? The jury is out on the first question until Nov. 4. But we now have the unambiguous answer to the second: Yes.

McCain, who is no racist, turned to this desperate strategy only as Obama started to pull ahead. The tone was set at the Republican convention, with Rudy Giuliani’s mocking dismissal of Obama as an “only in America” affirmative-action baby. We also learned then that the McCain campaign had recruited as a Palin handler none other than Tucker Eskew, the South Carolina consultant who had worked for George W. Bush in the notorious 2000 G.O.P. primary battle where the McCains and their adopted Bangladeshi daughter were slimed by vicious racist rumors.

Image Frank Rich Credit... Fred R. Conrad/The New York Times

No less disconcerting was a still-unexplained passage of Palin’s convention speech: Her use of an unattributed quote praising small-town America (as opposed to, say, Chicago and its community organizers) from Westbrook Pegler, the mid-century Hearst columnist famous for his anti-Semitism, racism and violent rhetorical excess. After an assassin tried to kill F.D.R. at a Florida rally and murdered Chicago’s mayor instead in 1933, Pegler wrote that it was “regrettable that Giuseppe Zangara shot the wrong man.” In the ’60s, Pegler had a wish for Bobby Kennedy: “Some white patriot of the Southern tier will spatter his spoonful of brains in public premises before the snow falls.”