It became such a refrain during that campaign that Mr. Obama found himself constantly reassuring supporters even as some of his aides fretted that his possible vulnerability would discourage some blacks from voting for him. “I’ve got the best protection in the world,” Mr. Obama reassured supporters who brought up the issue. “So stop worrying.”

The Secret Service did detect a spate of threats around the time Mr. Obama won the presidency and took office. But without providing numbers, the agency flatly denied reports that he had received three or four times as many as other presidents and added that they eventually subsided. “After his first election, there was a spike in his numbers,” Mr. Donovan said. “They’ve leveled out and they’ve been consistent and similar to his predecessors.”

For many blacks, the recent series of missteps by the agency charged with his protection has echoed powerfully. “One of the greatest fears of a first black president was harm being done to him,” Roland Martin, the talk show host, said on his radio program this week. The ability of a fence-jumper to make it all the way into the White House brought that home. “I can understand one or two, but for five layers to fail in the White House of all places?”

Charles D. Ellison wrote on The Root, a black-oriented online news site, that the episodes raised questions. “There could be only two reasons that Secret Service protection for President Barack Obama is slipping these days,” he wrote. “Either agents missed the memo that he’s the first black president or they really are just that overwhelmed.”

Such sentiments are not uncommon. Joshua DuBois, a former White House aide to Mr. Obama, said the president’s security feels personal for many blacks. “There’s a broad extended family around the country of moms and aunts and uncles who feel a real sense of kinship with this first family, and they want to make sure they’re protected and whole,” he said. “So you see a lot of concern right now.”

Some supporters of the president have long asserted that he has been treated with less respect by political opponents and the media, attributing that to his race. That has fueled suspicion that perhaps the Secret Service has not been as committed to him either.

Donald W. Tucker, one of the first black members of the Secret Service, who retired from the agency in 1990 and wrote a book about his experiences, said that he had no reason to believe the agency had not protected Mr. Obama vigorously but that he heard the concern regularly.