On April 4, I was at the Madame Walker Theater in Indianapolis for the premiere of  A Ripple of Hope, a documentary about an extemporaneous speech that Robert F. Kennedy delivered in Indianapolis 40 years earlier to a largely black audience just hours after Martin Luther King Jr. was assassinated. The moderator of a panel discussion that followed was a distinguished member of the Indianapolis African-American community. He began his comments by saying, “I know what are a lot of you are thinking…you’re thinking that the same thing that happened to Bobby could happen to Barack.” An audience that was about one-third African-American greeted this with gasps, then with shouts of “That’s right!” and “Yes!” The moderator explained that when he canvassed black neighborhoods for Senator Obama, people sometimes told him that they were afraid that if they voted for Obama, and he won the nomination, someone would shoot him.

The people in the audience understood something that seems to have escaped Sarah Palin and John McCain: that Obama, like Bobby Kennedy, faces a heightened risk of an assassination, a fact tacitly acknowledged by the extraordinary protection being afforded him by the Secret Service. Kennedy understood that his position as the oldest surviving brother of an assassinated president made him a target; several weeks before his death, he told author Romain Gary, “I know that there will be an attempt on my life sooner or later. Not so much for political reasons, but through contagion, through emulation.” Obama faces a similar risk. Not only because he is the first African-American to win a party’s nomination, but because he is also the most popular and charismatic black leader since the Rev. Martin Luther King, who was gunned down just two months before Bobby Kennedy—factors you might think would cause his opponents to be careful about how they attack his character and motives.

This is an incendiary charge to make seven years after real terrorists killed thousands of Americans, one that risks encouraging extremists to consider Obama fair game—and his assassination as a victory in the war on terror.

The line between a political smear that is merely unpleasant and one that is dangerously inflammatory is a matter of judgment, difficult to calibrate, and different depending on the candidate. Justice Potter Stewart’s famous statement is probably the best standard for rendering a judgment. I would argue that Governor Palin crossed the line when she accused Obama of being “not someone who sees America as you and I do—as the greatest force for good in the world…[but] someone who sees America as imperfect enough to pal around with terrorists who targeted their own country.” This is an incendiary charge to make seven years after real terrorists killed thousands of Americans, one that risks encouraging extremists to consider Obama fair game—and his assassination as a victory in the war on terror. Even Senator McCain seems to recognize that Palin has crossed a dangerous line. On Monday he asked an audience at a rally in Albuquerque, “Who is the real Barack Obama?” The first and loudest response came from a man who bellowed, “Terrorist!” On hearing this, McCain appeared to grimace.

Palin was only four years old when Bobby Kennedy was gunned down, but McCain is old enough to remember that Kennedy was demonized before he was assassinated. William Loeb, the conservative owner and publisher of the Manchester Union Leader, called him “the most vicious and dangerous leader in the United States today,” and after Kennedy announced his candidacy for the Democratic nomination, Mayor Richard Daley of Chicago likened him to Judas Iscariot. Earlier, the right-wing columnist Westbrook Pegler had welcomed the possibility that, as he put it, “some white patriot of the Southern tier will spatter [Kennedy’s] spoonful of brains in public premises before the snow flies.” (Pegler, it should be noted, is the anonymous writer whom Palin credited in her acceptance speech with saying, “We grow good people in our small towns, with honesty and sincerity and dignity.” The citation left Robert Kennedy Jr. aghast.)

Pegler’s remark was clearly beyond the pale, although he was only voicing the secret hopes of Kennedy enemies such as J. Edgar Hoover’s deputy Clyde Tolson, who at a gathering of senior FBI officials in April 1975 would say, “I hope that someone shoots and kills the son of a bitch.” But what about Loeb and Daley and other Kennedy enemies whose words were similarly incendiary? Were their attacks within the bounds of the rough and tumble of a political campaign, or did they cross a line? Did they risk pushing someone who was already mentally unbalanced over the edge, inciting an assassination? After all, if you believed that Robert Kennedy was “vicious and dangerous,” an American Judas, why not kill him before he became president?

All of us, Palin presumably included, pray that nothing happens to Obama between now and the election, and that Palin’s dangerous remarks will be remembered as simply one more smear in a campaign that may set a record for low-road political rhetoric. But if someone makes an attempt on Obama’s life in the coming months, Palin may have more to explain than why she was for that Bridge to Nowhere before she was against it.

Thurston Clarke is the author, most recently, of The Last Campaign: Robert F. Kennedy and 82 Days That Inspired America.