The use of the term dates back to around 1900, when it was used to describe Southern Democrats, and it became popularized during the 1928 Presidential contest. Many Democrats in the South had lingering resentments against the Republicans left over from the period of Reconstruction after the Civil War, and as a result they decided to stand behind the Democratic party, sometimes at all costs, rather than supporting Republicans. The yellow dog Democrats were a serious force to be reckoned with in the American south as a result.
The term is a reference to the “yaller dog,” both the term for a generic Southern dog and a specific breed of dog which is widely used in the South for hunting. The implication behind the term “yellow dog Democrat” is that someone would rather vote for a mangy yaller dog than a Republican candidate. The yellow dog Democrats ensured a strong showing in Southern elections for decades, until reformations in the party and national politics began to sway the balance more towards Republicans, but the voting base of Southern Democrats continues to be quite strong, and it is often heavily courted by Democratic candidates during the primaries.
Although the term was originally used somewhat pejoratively, some Democrats have embraced it, using it as affectionate slang and a term of endearment. Loyal Democrats can even purchase yellow dog gear to proclaim their loyalty to the party, and the term is referenced by the Blue Dog Democrats, a conservative coalition of Democrats in the House of Representatives on Capitol Hill.
Depending on how one views party loyalty, yellow dog Democrats could be considered both good and bad. On the good side, yellow dog Democrats often donate substantially to the party and urge friends to vote, upping voter turnout for the party. However, blind obedience is not necessarily a desirable trait, especially in politics, and political parties often benefit from internal dissent, developing new policies and approaches in response to such dissent and in turn becoming stronger.
