AARP is basically a welfare agency and cheerleading club for the democratic party and their agenda. With that in mind, one should spend little time wondering why they support OBama. There have been conservatives that have dropped out of AARP, but they are not losing sleep over the missed members. I understand that their membership is in the 10s of millions. If they are registered as a Political Action Committee, they would be the largest in the country.
Many years ago I decided not to join the AARP because they were on the NRA's list as anti 2A supporters. Their support of heath care reform that leaves seniors with less than desired care only shows they are not the defenders of the aging as they present themselves. They are a political lobbying agency with an agenda of their own.