But an intriguing sliver of his family history has received almost no attention until now: It appears that forebears of his white mother owned slaves, according to genealogical research and census records.
The records - which had never been addressed publicly by the Illinois senator or his relatives - were first noted in an ancestry report compiled by William Addams Reitwiesner, who works at the Library of Congress and practices genealogy in his spare time. The report, on Reitwiesner's Web site, carries a disclaimer that it is a "first draft" - one likely to be examined more closely if Obama is nominated.
According to the research, one of Obama's great-great-great-great grandfathers, George Washington Overall, owned two slaves who were recorded in the 1850 census in Nelson County, Ky. The same records show that one of Obama's great-great-great-great-great-grandmothers, Mary Duvall, also owned two slaves.
The Sun retraced much of Reitwiesner's work, using census information available on the Web site ancestry.com and documents retrieved by the Kentucky Department for Libraries and Archives, among other sources. The records show that Overall, then 30, owned a 15-year-old black female and a 25-year-old black male, while Mary Duvall, his mother-in-law, owned a 60-year-old black man and a 58-year-old black woman. (Slaves are listed in the 1850 census by owner, age, "sex," and "colour," not by name.)
An Obama spokesman did not dispute the information and said that the senator's ancestors "are representative of America."
"While a relative owned slaves, another fought for the Union in the Civil War," campaign spokesman Bill Burton said last night. "And it is a true measure of progress that the descendant of a slave owner would come to marry a student from Kenya and produce a son who would grow up to be a candidate for president of the United States."
The research traces the Duvalls to Mareen Duvall, a major land owner in Anne Arundel County in the 1600s. The inventory of his estate in 1694 names 18 slaves, according to a family history published in 1952.
The records could add a new dimension to questions by some who have asked whether Obama - who was raised in East Asia and Hawaii and educated at Columbia and Harvard - is attuned to the struggles of American blacks descended from West African slaves.
"The twist is very interesting," said Ronald Walters, a political scientist who is director of the African-American Leadership Institute at the University of Maryland, College Park. "It deepens his connection with the experience of slavery, even if it deepens it on a different side of the equation."
Gary Boyd Roberts, a senior research scholar at the New England Historic Genealogical Society who published a book on the family lineage of presidents, said he did not think the slave-holding history was "particularly unusual."
"If you have a white Southern mother, or a mother from the middle states who has ancestry in the South, it doesn't strike me that that should be very surprising," he said. While the majority of such families did not own slaves, many with some wealth did, Roberts said.
Reitwiesner's research identifies two other presidential candidates, Republican Sen. of Arizona and former Democratic Sen. of North Carolina, as descendants of slave owners. Three of McCain's great-great-grandfathers in Mississippi owned slaves, including one who owned 52 in 1860. Two ancestors of Edwards owned one slave each in Georgia in 1860.
It was unclear last night whether Obama was aware of any slave-holding ancestors, but he makes no mention of them in his 1995 memoir, Dreams from My Father: A Story of Race and Inheritance.
The book contains many approving references to his mother's side of the family. At one point, he writes that his mother "could give voice to the virtues of her midwestern past and offer them up in distilled form."
The memoir, however, comes close to confirming the Overall-Duvall lineage - stopping a generation short. Census and other records complete the gaps.
In a reference to his American ancestry, Obama writes "while one of my great-great-grandfathers, Christopher Columbus Clark, had been a decorated Union soldier, his wife's mother was rumored to have been a second cousin of Jefferson Davis, president of the Confederacy."
Clark was actually Obama's great-great-great-grandfather, according to Reitwiesner's research and census data available at ancestry.com. A 1930 census document lists Clark, 84, living in the same El Dorado City, Kan., household as a 12-year-old great-grandson, Stanley A. Dunham. Dunham was Obama's grandfather.
Clark's wife, Susan, was the daughter of George Washington Overall, the latest known family slave owner.
Reitwiesner, the researcher, declined to be interviewed for this article. "I'll let my Web page [wargs.com] speak for itself," he said in an e-mail. The Obama report contains a disclaimer that appears on all of Reitwiesner's work: "The following material ... should not be considered either exhaustive or authoritative, but rather as a first draft."
Genealogical experts who reviewed the Obama family tree at the request of The Sun would not vouch for its findings.
Most of the historical entries lack citations of authenticating source material, such as birth and death certificates or marriage licenses, said Barbara Vines Little, past president of the National Genealogical Society in Virginia, adding that "he has nothing here that I can see that would allow you to make any logical link."
"You just can't casually throw some documents together and make a sophisticated analysis," said Tony Burroughs, author of Black Roots: A Beginners Guide to Tracing the African American Family Tree and a consultant on a project by the New York Daily News that found that relatives of former Sen. Strom Thurmond appear to have owned the ancestors of civil rights activist the Rev. Al Sharpton.
But Roberts, the New England scholar, collaborated with Reitwiesner on a 1984 book about the American roots of Princess Diana, and calls him "exceptionally bright" and "quite a good researcher."
The online Obama family tree, Roberts said, is the work of "an informal team of genealogists" who specialize in Internet-based research and post their findings to test their validity.
"When you are gathering up things from the Internet, you can get fantastical - by that I mean wild and unbelievable - connections," Roberts said. "Many of them will fall; only a few of them will hold up. But some absolutely extraordinary things do hold up."
Assisting in the Obama research was Christopher Challender Child, a genealogist at the New England Historic Genealogical Society, who said that Reitwiesner culled fragments from a variety of sources. "There's a limit to what you can verify without a lot of money," Child said. "But from what I see, the line looks pretty good."
For some, the records may underscore Obama's unique racial heritage as a presidential candidate.
Author and essayist Debra J. Dickerson wrote in a January salon.com article that she had previously refrained from opining about the senator because "I didn't have the heart (or the stomach) to point out the obvious: Obama isn't black."
" 'Black,' in our political and social reality, means those descended from West African slaves," Dickerson said.
Walters, who was deputy campaign manager for Jesse Jackson in 1984 and the author of Black Presidential Politics in America, agreed that questions raised by Dickerson and others "is an important debate."
"What people are really asking is, 'Can I trust this guy? Do I have confidence in this guy? Does he understand my situation, and therefore [is he] able to take my issues into the political system?'" Walters said.