As Sen. Hillary Clinton, D-N.Y., arrived in Baton Rouge, La., Louisiana State University student Richard Wargo, 19, was in jail on $1 million bond for threatening the candidate.
A classmate told police Wargo threatened the former first lady and presidential candidate and talked of "burning down Clinton headquarters." LSU Police Chief Ricky Adams told The Associated Press that Wargo said "something along the lines of, 'Have you ever heard of a dead president?" an apparent reference to Clinton.
Wargo's friends opened a "Facebook" Web page in his defense, saying he was merely joking.
The 2008 race is a historic campaign. Clinton would become the first female president. Sen. Barack Obama, D-Ill., would become the first black president. Both have faced recent threats against them.
As a former first lady, Clinton already has federal Secret Service protection. Most major candidates receive a Secret Service detail 120 days before the general election. But on Thursday, Obama became the second candidate to gain one.
Illinois' senior senator, Dick Durbin, said he requested the protection for fellow Democrat Obama after hearing of "disturbing" threats against the candidate.
"It is a sad reality in this day and age that Mr. Obama's African-American heritage is a cause for very violent and hated reaction from some people," Durbin said Thursday.
It's a risk faced by other black candidates, including Al Sharpton and Jesse Jackson.
"When I announced in '84, the very first day they had to bring out the Secret Service because we received the most threats of any candidate at that time in American history," Jackson said. "The hate is unfounded but the violence is real. ... When you're drawing big crowds, all kinds of elements [are] in those crowds -- some are cheering and some are stalking."
Threats to candidates led the Washington Post editorial page to urge Department of Homeland Security Secretary Michael Chertoff to take action, saying, "With an estimated $100 million already allocated for this purpose, Mr. Chertoff should consider bringing more into the fold sooner rather than later."
The history of assassinations in the United States is a violent one. Four presidents -- Lincoln, Garfield, McKinley and Kennedy -- have been assassinated. Theodore Roosevelt, Franklin Roosevelt, Harry Truman, Gerald Ford and Ronald Reagan all survived assassination attempts.
"The crowds are important for the candidates for a political reason, but they make security coverage -- especially the agents -- cringe," said former Secret Service agent Joe LaSorsa, who once protected Reagan.
Gunmen have attacked only two candidates. A triumphant Robert Kennedy had just won the California Democratic primary in 1968 when he arrived at the Ambassador Hotel in Los Angeles. The image of his death -- captured in a photo as a horrified campaign worker looked on just five years after Kennedy's brother, President John Kennedy, was assassinated -- is burned into the national memory.
Four years later, segregationist Alabama Gov. George Wallace was paralyzed by a gunman's shot in Maryland.
Since then, Secret Service protection has been available on request to all presidential candidates. But they have to pass a review that measures their prominence, based on polls and fund-raising.