Mr. Obama was 6 at the time, and like many of his admirers, he has only read about the violence that traumatized the nation. But those recollections and images are often invoked by older voters, who watch his candidacy with fascination, as well as an uneasy air of apprehension, as Democrats inch closer to selecting their nominee.

Mr. Obama has had Secret Service agents surrounding him since May 3, the earliest a candidate has ever been provided protection. (He reluctantly gave in to the insistent urging of Senator Richard J. Durbin, Democrat of Illinois, and others in Congress.) As his rallies have swelled in size, his security has increased, coming close to rivaling that given to a sitting president.

His wife, Michelle Obama, voiced concerns about his safety before he was elected to the Senate. Three years ago, she said she dreaded the day her husband received Secret Service protection, because it would mean serious threats had been made against him.

Among friends and advisers, danger is something Mr. Obama rarely mentions.

“It’s not something that I’m spending time thinking about day to day,” said Mr. Obama, who has been given the Secret Service nickname Renegade, a way for agents to quickly identify him. “I made a decision to get into this race. I think anybody who decides to run for president recognizes that there are some risks involved, just like there are risks in anything.”

Not long ago, his advisers worried that some black voters might not support his candidacy out of a fierce desire to protect him. It was a particular concern in South Carolina, but Mr. Obama said he believed the worry was also rooted in “a fear of failure.”