"...the security measures were developed with Dallas-area law enforcement personnel.

Now that's a reassurance that just sends shivers up the spine, doesn't it, with its echoes of 1963.
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..."the thought that this guy might be in danger has surely crossed the minds of a number of people but it ain't one I like to see in print, even on this site." Does printing it on screen makes it more likely to happen? That strikes me as close to magical thinking.

Obviously there's a real risk. A larger risk than the one that any President or presidential candidate has. Undoubtedly Obama knows that, and has considered it, and decided he wouldn't let it stop him doing what he felt was the right thing. That's the only way to deal with this aspect of American politics, not to pretend it's not there, but to face it and to set it aside.