You may know him better as "Leo" from The West Wing.
Tragically, it would seem that life, indeed, does imitate art. His character suffered a heart attack on the show during the sixth season. He recovered.
I don't know how the show will deal with the loss. Presumably (although I don't know for sure) the show has finished taping all the remaining episodes for this season. But if Jimmy Smits' character is elected President, Leo would have been the VP.
Of course, there's a good chance this may be the last season for TWW anyway, in which case it wouldn't matter.
At any rate, tonight I will raise a glass in John's memory. Farewell! We'll miss you!