Theresa, one of the folks who comes weekly to watch "The West Wing" at my place, called me from her mom's place in Pennsylvania to share the sad news! I would have found out sooner or later, as several of you have already posted about it.
I can only imagine "Leo" will die of a heart attack after they use up the episodes they now have in the can, and someone else will be appointed vice president. Not sure whether they have already shot the election night episode.
He was in loads of other roles, as well... odds are you've seen him.
* News stories