The article quotes independent pollster Peter Hart:
"The big question about Barack Obama from the very beginning has been, Is he safe? . . . . Safe in terms of both the cultural values that he has, and about whether he is strong enough to be commander in chief."
I think Hart is on to something, but neither he nor the article articulate the other dynamic: religion.
Specifically, I think it is far more pervasive than talked about in the news media that Americans believe in their heart-of-hearts that Obama is Muslim. So, Hart is right that people want to know if he's "safe," but with regard to whether or not he's really one of "them," one of our enemies, the people we're fighting "over there", the people who attacked us on 9/11.
This notion of his Muslim roots crops up everywhere. A church in South Carolina posted on their church sign the following:
Obama Osama Hmmmm. Are they brothers?
When asked about the sign, the pastor of the church said that he didn't mean to offend anyone, he just thought it was worth thinking about the connection, suggesting that they were "brothers" because of a shared faith.
My own neighbors ask me whether I think that his "past" and his "background" will surface during an Obama presidency. When I asked what they were referring to, they mentioned Obama's Muslim father and step-father, his education in Indonesia at a "madrasa" (another name for school, by the way, not ultra fundamentalist, Islamist school). When I explained what I knew of Obama's background and his current alignment with the Christian faith, they were still unconvinced that Obama was really who he seemed to be.
So, I think another strong undercurrent that explains Clinton's continued success and Obama's failure to win over a specific demographic in the Democratic party, has as much to do with religion as it does race.