I'm just stunned. Those of you who follow the show will know that John's character had survived a heart attack and surgery previously, and is now the VP candidate in an election due in a few weeks. I can't imagine what they're going to do or how far in advance they have filmed. They must be blown away on both personal and professional levels. It sounds like he didn't know he had heart disease.
Rest in Peace--you were a hell of an actor.