"The Secret Service will be monitoring Facebook and other social media," the poster cautioned others as they planned a disruption inside a Southern California rally for the presumptive GOP nominee. "Be smart when posting. Any perceived threats will come with a knock at the door."

If the Secret Service was monitoring that conversation, it isn't saying. But combing through social media for potential threats amid one of the most divisive presidential campaigns in memory ultimately falls to the agency, as does providing in-person protection to Trump, Sanders and likely Democratic nominee Hillary Clinton. It may be too much, even for an agency with a $2 billion budget and 3,200 special agents, say some experts.

"The Secret Service has a very difficult task," said James Reese, founder and chairman of leading security firm TigerSwan. "You can only secure so much, and the men and women in the agency work their butts off.

Michael Steven Sandford was arrested after trying to grab a police officer's gun. He told the Secret Service he wanted to kill Donald Trump. (Associated Press)

A failed attempt earlier this month by a British national to assassinate Trump reminded the public of the ever-present danger lurking at every rally, campaign stop and fund-raising dinner. Michael Sandford, 20, was arrested after trying to grab a cop's gun, which he allegedly told the Secret Service he had hoped to use to kill the developer.

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