I thought this was an interesting new tidbit from a recent article on a Secret Service man on JFK's detail in Fort Worth, the stop before the Dallas:
During his advance work, Howard had investigated approximately 30 people who had made some kind of threatening comment.He told me they came up with whatever reason they could dream up to put some of them in jail during the visit. Many of them disliked Kennedy solely because he was Catholic. “Some of them made the crack that they ought to kill that SOB,” said Howard. Of those 30, some ended up in jail and others tailed by agents and police.