http://www.c-span.org/questions/weekly55.htm In the 106th Congress, the Blue Dogs are 30 fiscally conservative House Democrats who tend to vote together as a coalition on budgetary and economic issues. Their stated goal is to bring their own party back to the center of the ideological spectrum, and to forge good working relationships with moderate Republicans to help move that party more toward a centrist agenda.
They have been most influential since their creation in the 104th Congress because of the leverage they exert as a unified voting bloc. The Republican leadership, with 223 Members on its side of the aisle, has only 5 votes to spare to meet the minimum 218 required to pass legislation If more than 5 Republican Members stray from the fold on any given vote, the leadership often turns to the Blue Dogs on the Democratic side of the aisle to gain their bloc vote in exchange for negotiated changes in legislative language.
The Blue Dogs derive their name from the artwork of a Cajun painter, George Rodrigue, well known in Louisiana for his series of paintings featuring an unusual blue dog. The fledgling members of what became the Blue Dog Coalition used to meet regularly in the offices of then-Democrats Rep. Billy Tauzin and Rep. Jimmy Hayes of Louisiana. Tauzin has since switched to the Republican party and Hayes was defeated in a run for the Senate. The Louisiana representatives had Rodrigue’s blue dog paintings displayed on the walls of their offices, and these provided the inspiration for the coalition’s name. One of the Blue Dogs, Rep. John Tanner from Tennessee, maintains that Blue Dogs are simply “yellow dogs that have been choked by extremes in both political parties to the point they have turned blue.”
Blue Dog Democrats are an actual voting coalition made up of Members of Congress, whereas Yellow Dog Democrat is an expression -- it describes a certain kind of voter. Nor are Blue Dogs ideological relatives of the "Yellow Dogs" of the South, even though they have similar names.