These incidents are deplorable, awful and totally wrong. Anyone who is refusing to vote for Barack Obama just because he's black is a racist, and should be ashamed. (And how shocking to learn that this is going on in a Democratic primary . . . haven't we always been instructed that the Democrat Party is the party of the enlightened?!).
There's a lurking danger in these kinds of narratives, as well. When stories like these go into wide circulation, it's a sure bet that some on the left are going to start asserting -- not that America is a great country with (unfortunately) some racists (all of which is true) -- but that America is a predominantly racist country with some great people (i.e., those who support Barack Obama). If Barack loses in the fall, the USA will officially be labeled a racist country by many of its left-wing citizens.
Back in the realm of reality, it's the sad truth that there will probably be some people who won't vote for Barack just because of his skin color. But there will be many, many more who oppose him because of his leftist policies and his radical associations. It's important to distinguish between the two. And that's not just because it's wrong both to slander all GOP voters by calling them racists and to equate all opposition to Barack with racism per se. It's also important for the sake of the candidate and the campaign itself that the distinction between policy opponents and invidious-motive opponents be grasped (as I noted here when Hillary Clinton supporters were crying "sexism!").
Finally, I've done my share of phoning and even a little door-to-door stuff in the past. My candidates happened to be white males, but even then, some of the epithets were pretty ugly. It would be interesting to know whether any of the people recounting these stories with evident shock and horror are campaign veterans, or first-timers who haven't been exposed to the rough and tumble of grassroots work. Racist epithets are always unacceptable and wrong, but as campaign veterans can attest, at some point, unfortunately, some people will always resort to the ugliest kind of slurs, whether they're anti-female, anti-minority or anti-gay.