Sen. Barack Obama's candidacy is forcing America to examine the complexities and peculiarities of race in this country -- including the lingering power of the historic One-Drop Rule.

Simply stated, the One-Drop Rule said that a person with any known African ancestry -- even as little as one drop of African blood -- was considered black, regardless of his or her appearance.

"We're one of the few countries in the world that had this One-Drop Rule -- even South Africa didn't have it," said Laurence Glasco, Ph.D., an associate professor who teaches about race and ethnicity in the University of Pittsburgh's history department.

The rule, a social construct and vestige of slavery, was designed to discourage interracial relationships and preserve the "purity" of the white race. By the mid-1920s, most states had adopted it in some form until the U.S. Supreme Court ruled it unconstitutional in 1967.

Remnants of the One-Drop Rule are still evident in America's psyche. Although Mr. Obama is biracial -- his father was a black man from Kenya and his mother was a white woman from Kansas -- many view him as black and he identifies himself as such.

In recent years, however, some people have seemed more open to acknowledging that biracial people and multiracial people are more than one race and not only the race they appear to be.

In November 2006, Mark Williams had Zogby International conduct an Internet poll of more than 2,000 people, telling them Mr. Obama's parents' heritage, then asking them "What race is Sen. Obama?"

Only 7.9 percent of whites identified Mr. Obama as black compared with 8.9 percent of Hispanics and 8.3 percent of Asians, said Mr. Williams, author of "The 10 Lenses: Your Guide to Living & Working in a Multicultural World."

By comparison, more than 65 percent of African Americans surveyed identified Mr. Obama as black. And more than 75 percent of whites identified Mr. Obama as biracial or multiracial compared with only 22 percent of African Americans.

Despite the polling results, Mr. Williams believes the One-Drop Rule still is very much in effect in American society.

"I think a lot of the data that we collected was aspirational," says Mr. Williams, an organizational psychologist based in Bethesda, Md. "It can't be 100 percent accurate, but the great thing about it is that people know what the right answer should be."

Although most people see Mr. Obama as black, consciously or subconsciously some voters may be drawn to him because he is half white.

"The reason [whites] feel comfortable with Obama is that he's 'part one of them,' " said G. Reginald Daniel, Ph.D., an associate professor of sociology at the University of California, Santa Barbara, and author of "More than Black?: Multiracial Identity and the New Racial Order."

"People are most comfortable with things they see as like themselves," he says. "If [white voters are] going to feel comfortable with an African American, they're going to feel comfortable with someone who is more like them."

Mr. Obama has all the markers of whiteness, he says, including his Ivy League education.

"If he didn't have a white mother, [whites] might still resonate with him because of the way he looks," Dr. Daniel said. "He's a prince."

Mr. Obama also isn't biologically connected to the black American history of slavery.

Culturally, however, people still are reluctant to use the terms biracial and multiracial.

"Barack grew up in a time when being multiracial or biracial wasn't an option and you had to make a choice," Mr. Williams said.

Biracial and multiracial people born within the past 20 years haven't felt as much pressure as earlier generations of multiracial people to choose one racial identity over another, Dr. Daniel said. Some still do choose, but some see themselves as part of a unique, specific mixed-race group.

The multiracial movement grew between 1990 and 2000, when the U.S. Census first allowed people to identify themselves as belonging to more than one race.

Multiracialism remained in the news, fueled by the arrival of professional golf phenom Tiger Woods, and later the revelation that Essie Mae Washington-Williams was the secret, biracial daughter of the late South Carolina Sen. Strom Thurmond, a longtime segregationist.

The movement quieted for a time. Then, Mr. Obama came along and reinvigorated the multiracial movement, Dr. Daniel says.

"A lot of the biracial-multiracial identity stuff comes down to color," Mr. Williams says. "If you are dark, you are going to be treated externally in the world as if you're black or a person of color. If you're light, you have more options."

In 1890, the U.S. Census included the racial categories of mulatto (a person with one white parent and one black parent), quadroon (a person one-quarter black) and octoroon (a person one-eighth black).

There was a backlash against mulattoes during around the time of Jim Crow, segregation and lynching. Around 1900, mixed-race people, who were viewed with suspicion and considered an unhappy and troublesome bunch more prone to fight for their rights because of their partially white heritage, said Dr. Glasco.

"There was an effort to demonize the mulatto and part of it was to push them back into the general category of negro,' " Dr. Glasco said. The U.S. Census dropped the racial designations of mulatto, quadroon and octoroon in 1910.

Historically, lighter-skinned slaves were permitted to work in slave owners' homes, while darker-skinned slaves were restricted to working in the outdoor fields. Light-skinned slaves sometimes also received privileges denied to dark-skinned slaves.

Studies have shown colorism -- discrimination based on skin tone or a favoring of those with lighter skin tones -- remains today in America among people of all races.

Looking at a photograph of Mr. Obama with former Tennessee Congressman Harold Ford, Dr. Daniel was struck by the fact that Mr. Ford, who has two black parents, looked whiter than Mr. Obama, who is biracial.

"Obama and Ford represent a tradition that still exists in this country where blacks with white [or lighter] skin got opportunities first," he said.

In some ways, Mr. Obama is a human Rorschach inkblot test in the eyes of many Americans, says Joy M. Zarembka, author of "The Pigment of Your Imagination: Mixed Race in a Global Society."

"He appears to be what you want him to be and his multiracial background allows a cross-section of voters to feel comfortable," says Ms. Zarembka, who has a Kenyan mother and European-American father and grew up in Point Breeze and Squirrel Hill.

"Black voters that like Obama see his black side and white voters who like him see his white side," she says.

Conversely, for those who oppose him based on race, it's the opposite.

"He might seem too black for some white voters and too white for some black voters," says Ms. Zarembka, executive director of the Break the Chain Campaign at the Institute for Policy Studies, a progressive think tank in Washington, D.C.

Young people of all races are among some of his most ardent supporters. But more than a few exit polls this primary season have shown the vote breaking down along racial lines in higher age groups.

Dr. Daniel is concerned that some may view Mr. Obama's message of inclusiveness and his success thus far as a sign that race doesn't matter anymore in America and that society is color-blind.

"His one case does not mean we've done a thing about [racial inequalities in] income, life expectancy, healthcare, infant mortality rates or employment," he said.

He also believes that if white voters view Mr. Obama as "too black," his candidacy will be jeopardized.

"Public figures of color have felt they had to de-colorize themselves or dance around race to attract a larger constituency," Dr. Daniel said.

That, in part, may be why Mr. Obama last week publicly and forcefully broke ties with his former pastor, Rev. Jeremiah Wright, following Mr. Wright's recent controversial remarks during appearances on "Bill Moyers Journal" and in speeches before the NAACP in Detroit and the National Press Club in Washington. Mr. Obama soundly denounced and rejected the ideas espoused by Mr. Wright, whom he said he felt had disrespected him and his candidacy.

Mr. Obama also has distanced himself from outspoken civil rights activists, Revs. Jesse Jackson and Al Sharpton, and declined to appear at Tavis Smiley's "State of the Black Union" symposium in New Orleans in February or appear at the Lorraine Motel in Memphis on the anniversary of Dr. Martin Luther King Jr.'s assassination in April.

"He needs to be able to convey that he is respectful of that [civil rights] tradition of leadership, but differentiate himself from it," Dr. Daniel said.

Mr. Obama is trying mightily to straddle the racial line as a metaphoric bridge between various constituencies and his candidacy is allowing, if not forcing, America to have more honest conversations about race.

"He is making us look at our reflection in the mirror," Ms. Zarembka says. "The question is whether we're brave enough to look back."

L.A. Johnson can be reached at ljohnson@post-gazette.com or 412-263-3903.