I would think that, while a Priest could never reveal anything confessed to him, he could interfere with intentions confessed to him if it in no way broke the seal of confidence. For instance, the Priest could perhaps call the nearest authorities capable of helping, from a payphone (so that even his identity as a priest could never be traced) and say something like “Protect the President…oh please protect the President. I…I can’t say why, but you’ve got to do it!” Such an anonymous, simple warning lets the authorities know something’s wrong…they’ve no clue what’s the matter, but they know enough so that, if they’ll listen, they will prevent the murderer’s attack.
Furthermore, in this scenario, the Priest has done nothing he wouldn’t possibly (and morally) do without having heard the confession. He might randomly decide that the President needs protection; he might have just as easily made the same, lie-free phone call without ever having known that anyone actually intended to hurt the President. Nothing would’ve been immoral in him doing so, if his concern was genuine; I can’t imagine the confession changing that and limiting him from doing something he was perfectly free to do just moments before the confession.
Of course, I could be way off, and the Priest may not be allowed to even do that. However, this is the best I can think of, as the Priest would not be revealing anything that he actually learned in confession. One doesn’t necessarily have to know anyone’s in danger before making such a warning/plea.
If I’m completely wrong (after all, I’ve never read Canon Law), someone please do two things: 1. Correct me. 2. Tell us what the priest could do, if anything, if faced with such a situation.