Department of Homeland Security Secretary Michael Chertoff authorized Obama's protection after consultations with the bipartisan congressional advisory committee, according to Chertoff spokesman Russ Knocke and the Secret Service.
Obama, who frequently draws crowds in the thousands at campaign stops, requested the protection. Secret Service spokesman Eric Zahren and Department of Homeland Security officials said they were not aware of any threats to the Illinois senator.
According to a senior law enforcement official, the security detail was prompted by general concerns about the safety of a prominent black candidate. Although there was no direct threat to Obama, several factors raised concerns, including some racist chatter on white supremacist Web sites.
The official spoke on condition of anonymity because of the security issue.
Obama's rival, Sen. Hillary Rodham Clinton, D-N.Y., has a Secret Service detail that is provided to all former first ladies.
In the last election, Democratic candidates John Kerry and John Edwards received their protection in February 2004 as they were competing for the party's nomination. Obama's detail comes nine months before the first votes are cast.
Federal law allows candidates to seek protection if they meet a series of standards, including public prominence as measured by polls and fund raising. The members of the congressional advisory committee are the Speaker, the House and Senate majority and minority leaders as well as one additional member.
In a Feb. 12 interview with The Associated Press, Obama dismissed concerns about his own security, but would not answer directly when asked if he had received death threats. The Rev. Jesse Jackson drew early Secret Service protection because of violent threats during his campaigns for president in the 1980s.
"I face the same security issues as anybody," the Illinois senator told the AP. "We're comfortable with the steps we have taken."
The Secret Service and Homeland Security would not comment on the scope of Obama's protection or provide any other security details. Obama's campaign declined to comment.