IF you think way back to the start of this marathon campaign, back when it seemed preposterous that any black man could be a serious presidential contender, then you remember the biggest fear about Barack Obama: a crazy person might take a shot at him.

Some voters told reporters that they didn’t want Obama to run, let alone win, should his very presence unleash the demons who have stalked America from Lincoln to King. After consultation with Congress, Michael Chertoff, the homeland security secretary, gave Obama a Secret Service detail earlier than any presidential candidate in our history — in May 2007, some eight months before the first Democratic primaries.

“I’ve got the best protection in the world, so stop worrying,” Obama reassured his supporters. Eventually the country got conditioned to his appearing in large arenas without incident (though I confess that the first loud burst of fireworks at the end of his convention stadium speech gave me a start). In America, nothing does succeed like success. The fear receded.

Until now. At McCain-Palin rallies, the raucous and insistent cries of “Treason!” and “Terrorist!” and “Kill him!” and “Off with his head!” as well as the uninhibited slinging of racial epithets, are actually something new in a campaign that has seen almost every conceivable twist. They are alarms. Doing nothing is not an option.

All’s fair in politics. John McCain and Sarah Palin have every right to bring up William Ayers, even if his connection to Obama is minor, even if Ayers’s Weather Underground history dates back to Obama’s childhood, even if establishment Republicans and Democrats alike have collaborated with the present-day Ayers in educational reform. But it’s not just the old Joe McCarthyesque guilt-by-association game, however spurious, that’s going on here. Don’t for an instant believe the many mindlessly “even-handed” journalists who keep saying that the McCain campaign’s use of Ayers is the moral or political equivalent of the Obama campaign’s hammering on Charles Keating.

What makes them different, and what has pumped up the Weimar-like rage at McCain-Palin rallies, is the violent escalation in rhetoric, especially (though not exclusively) by Palin. Obama “launched his political career in the living room of a domestic terrorist.” He is “palling around with terrorists” (note the plural noun). Obama is “not a man who sees America the way you and I see America.” Wielding a wildly out-of-context Obama quote, Palin slurs him as an enemy of American troops.

By the time McCain asks the crowd “Who is the real Barack Obama?” it’s no surprise that someone cries out “Terrorist!” The rhetorical conflation of Obama with terrorism is complete. It is stoked further by the repeated invocation of Obama’s middle name by surrogates introducing McCain and Palin at these rallies. This sleight of hand at once synchronizes with the poisonous Obama-is-a-Muslim e-mail blasts and shifts the brand of terrorism from Ayers’s Vietnam-era variety to the radical Islamic threats of today.

That’s a far cry from simply accusing Obama of being a guilty-by-association radical leftist. Obama is being branded as a potential killer and an accessory to past attempts at murder. “Barack Obama’s friend tried to kill my family” was how a McCain press release last week packaged the remembrance of a Weather Underground incident from 1970 — when Obama was 8.

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.

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.”

This is the writer who found his way into a speech by a potential vice president at a national political convention. It’s astonishing there’s been no demand for a public accounting from the McCain campaign. Imagine if Obama had quoted a Black Panther or Louis Farrakhan — or William Ayers — in Denver.

The operatives who would have Palin quote Pegler have been at it ever since. A key indicator came two weeks after the convention, when the McCain campaign ran its first ad tying Obama to the mortgage giant Fannie Mae. Rather than make its case by using a legitimate link between Fannie and Obama (or other Democratic leaders), the McCain forces chose a former Fannie executive who had no real tie to Obama or his campaign but did have a black face that could dominate the ad’s visuals.

There are no black faces high in the McCain hierarchy to object to these tactics. There hasn’t been a single black Republican governor, senator or House member in six years. This is a campaign where Palin can repeatedly declare that Alaska is “a microcosm of America” without anyone even wondering how that might be so for a state whose tiny black and Hispanic populations are each roughly one-third the national average. There are indeed so few people of color at McCain events that a black senior writer from The Tallahassee Democrat was mistakenly ejected by the Secret Service from a campaign rally in Panama City in August, even though he was standing with other reporters and showed his credentials. His only apparent infraction was to look glaringly out of place.

Could the old racial politics still be determinative? I’ve long been skeptical of the incessant press prognostications (and liberal panic) that this election will be decided by racist white men in the Rust Belt. Now even the dimmest bloviators have figured out that Americans are riveted by the color green, not black — as in money, not energy. Voters are looking for a leader who might help rescue them, not a reckless gambler whose lurching responses to the economic meltdown (a campaign “suspension,” a mortgage-buyout stunt that changes daily) are as unhinged as his wanderings around the debate stage.

To see how fast the tide is moving, just look at North Carolina. On July 4 this year — the day that the godfather of modern G.O.P. racial politics, Jesse Helms, died — The Charlotte Observer reported that strategists of both parties agreed Obama’s chances to win the state fell “between slim and none.” Today, as Charlotte reels from the implosion of Wachovia, the McCain-Obama race is a dead heat in North Carolina and Helms’s Republican successor in the Senate, Elizabeth Dole, is looking like a goner.

But we’re not at Election Day yet, and if voters are to have their final say, both America and Obama have to get there safely. The McCain campaign has crossed the line between tough negative campaigning and inciting vigilantism, and each day the mob howls louder. The onus is on the man who says he puts his country first to call off the dogs, pit bulls and otherwise.