I polled myself and I think you are right. I sent him a fax at the time saying, "Mr. President, don't you think it is time for Terri to have a sip of water"?.
I noticed the same drop in the polls at that time. I think he lost supporters on both sides. Those who value life were disappointed in the limited gesture he gave, and those who support forced euthanasia were angry at him for interfering at all. His poll numbers have steadily dropped since then.