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.
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.
Sen. Dick Durbin, D-Ill., told reporters Thursday night that several weeks ago he received information that made him concerned for Obama's safety. The number of people Obama was attracting at campaign events also worried him, Durbin said.
"Some of the other information that was given to us, unfortunately, I think, raised a concern among many of his friends," Durbin, said. "Unfortunately, some of the information that we found was racially motivated."
Obama, D-Ill., who frequently draws crowds in the thousands at campaign stops, requested the protection. However, Secret Service spokesman Eric Zahren and Homeland Security officials said they were not aware of any threats to the senator.
Durbin said he approached Senate Majority Leader Harry Reid, D-Nev., six to eight weeks ago and that Reid and Republican leader Mitch McConnell, R-Ky., went to the Secret Service last week.
"I expressed concern because of my affection for Barack and his family. I've traveled with Obama. I've witnessed enormous crowds," Durbin said. "This is a relief."
In a February interview with 60 Minutes' Steve Kroft, Obama's wife, Michelle, addressed the possibility that her husband could be the target of an assassination attempt. "I don't lose sleep over it because the realities are that, you know, as a black man, you know, Barack can get shot going to the gas station, you know," she said. "So you can't make decisions based on fear and the possibility of what might happen. We just weren't raised that way."
Obama campaign officials will not say why they requested early protection, reported CBS News correspondent Bob Orr. But one source inside the campaign said there is concern about the size of Obama's crowds, and the crush of people who want to press close or even touch a candidate who exudes rock star appeal.
The security is not cheap — more than $100 million has been budgeted
for protection for this campaign season — nearly a 50 percent increase over what was spent on security in 2004, Orr reported.
Sen. Hillary Rodham Clinton, D-N.Y., also a presidential candidate, 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 fundraising. 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 he would not answer directly when asked if he had received death threats. 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.