Agents must investigate any disturbing sign. Have any police uniforms been stolen lately in the area? Have gun sales surged? Hours before the event, bomb-sniffing dogs scour the site. As crowds or delegates file into an event, screeners carefully check each person.
On the campaign trail the advance work becomes even more demanding. Candidates may visit three different states in a single day.
Threats against the candidates increase as the election draws closer. Leading up to the 2000 election, agents saw a spike in the number of threats against the presidential candidatespartly attributed to a highly divisive political climate. This year is shaping up to be even worse.
"There is even more interest in the protectees, and that creates more challenges for the Secret Service," Stafford said. "More people are coming out to campaign events, and there is more to keep track of."
Death threats against protected government officials might be phoned in to a police department or a television station. E-mailed threats have become more common than threat letters.
"We investigate every threat that we receive," said Anthony M. Chapa, a U.S. Secret Service special agent based in Los Angeles.
At one point Chapa even received a personal death threat. The culprit was soon identified. When uniformed police showed up at his door, the letter-writer fell back and died.
President George W. Bush has visited the Los Angeles area twice this year, and Vice President Dick Cheney has been to the city a few times. But the most significant visit that Chapa has had to organize in the middle of the presidential election was the state funeral for former President Ronald Reagan.
"It was an immense undertaking," Chapa said. "No one on the job today had ever done a state funeral."
An incident involving President Reagan, incidentally, underlines the challenge that the Secret Service faces in uncontrolled environments. In 1981 John Hinckley fired six rounds at President Reagan as he exited a hotel in Washington, D.C. Special Agent Jerry Parr saved Reagan's life by covering and evacuating him.
"Agents are trained to cover and evacuate, and cover means cover with your own body," Parr said in the recent National Geographic documentary Inside the U.S. Secret Service. "It's a counter-instinctive movement. It's a muscle memory. And in a certain sense, I had been waiting for that all of my career, and it is a moment when you really think about it deeplywhere history and destiny hang in the balance."
On at least one occasion a President's decision to change plans almost caused a disaster. On September 5, 1975, President Gerald Ford had just given a speech at the California State Building in Sacramento when he decided to walk across a park where a crowd had gathered.
Shaking hands with the enthusiastic supporters, Ford noticed a woman in a red dress following him. Suddenly, she whipped out a gun. At that moment Special Agent Larry Buendorf instinctively stepped in front of Ford. Buendorf pulled the gun away and wrestled the woman to the ground and cuffed her.
The potential assassin turned out to be Lynette "Squeaky" Fromme, a member of the brutal Charles Manson gang.
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