I heard that too and I would guess any threats they are getting are from the other side with the hope of it being blamed on the conservatives. Not too many are trying to save Terri's life and trying to end someone else's. With caller ID being what it is today, any threats could easily be traced.
It ought to be a slam dunk to find out who was making threats. There is also the possibility that callers were saying they wouldn't vote for that person again, and of course that would be a threat to their agenda and pocketbook. They could be exaggerating what the 'threat' actually was.