Thompson was endorsed for re-election in 2008 by the Humane USA PAC, a political action committee founded by leaders of animal rights groups such as The Humane Society of the United States, The Fund for Animals, American Society for the Prevention of Cruelty to Animals, the Doris Day Animal League, the Animal Welfare Institute, Farm Sanctuary and The Ark Trust. Thompson has an 88% lifetime rating from the Defenders of Wildlife Action Fund, which endorsed him for re-election in 2008. This organization is the 501(c)(4) sister organization of Defenders of Wildlife.