A "yellow-dog" Democrat usually describes a voter-- a voter whose identity is so deeply tied to the Democratic party that he/she will vote for the Democratic candidate "even if it is a yellow dog."
A "blue-dog" Democrat is usually (ironically) a red state Democrat who believes the only way to get elected in predominantly conservative communities is to vote with Republicans as often as necessary, even 70% of the time or more.
Enter the new "green-dog" Democrat. Taking up the cause of the environment and championing progressive economic policies and liberal social policies, "green dogs" present a positive alternative to the sour pictures elicited by the former "dog" descriptions. They appropriate the language of disappointment and transform it into a moniker signifying of renewal, preservation of natural resources, popular resurgence and a sense of humor. Use the term to separate the best of the best from "stray dog" Democrats who have strayed too far from progressive and environmentally sound values.