By Bill Sammon

The dominant storyline in Barack Obama's presidential nomination this week is the historic shattering of a racial barrier, which could end up defining him in voters' minds as "the black candidate," thereby undermining Obama's effort to position himself as "post-racial."

"He definitely does not want to get locked into some category as a black candidate because that's almost a sure death knell for his campaign," said civil rights author Juan Williams of National Public Radio. "Being post-racial is what got him this far."

"There are a lot of people, black and white, who hope that he's the guy to heal all racial wounds and take us away from those unending racial tensions in the country, someone who doesn't focus exclusively on race," added Williams, a Fox News contributor. "I think there are lots of white people who say, ‘I feel good about supporting this black guy.'"

Professor Marc Lamont Hill of Temple University sees irony in Obama accepting the Democratic Party's presidential nomination on Thursday, the 45th anniversary of Dr. Martin Luther King's "I have a dream" speech, because Obama rarely discusses race.

"To be sure, Barack Obama was attempting to run a campaign that was marked by color-blindness and color muteness," Hill said. "You know, ‘I'm not going to run as a black candidate, I'm not going to talk about my blackness. I'm not going to ignore it, but it's not central to my candidacy. I don't want people to see me as black.'"

But all that changed in March, when Obama's incendiary pastor, the Rev. Jeremiah Wright, made racially inflammatory remarks that forced Obama to give a speech on race relations. Suddenly, Obama was anything but "post-racial."

"From that moment forward, Barack Obama has certainly been seen as a black candidate," Hill said. "But as the Jeremiah Wright thing died down, it's gone from the front of our conversation to the back."

That is, until this week, when Obama's nomination unleashed a deluge of media coverage about him making history by becoming the first black candidate to be nominated for president by a major party. That has prompted some to grumble that Obama still seems reticent to discuss civil rights issues.

"Why does the guy never say anything - anything -- about the most important racial issues, civil rights issues in the country, like school vouchers?" Williams said. "He never says anything, so people then think, ‘Well, maybe he's some secret militant and crazy.' That's what they think about his wife or about Reverend Wright."

Hill agreed.

"To some extent, Barack Obama's failure to speak more often about race makes people worry," he said. "Since he never talks about race, they don't know where he stands, so their minds start to turn against him."

Bill Sammon is the Washington deputy managing edtor of Fox News.