I got into a short exchange on Google+ with someone who says she's from another country - I don't know which. She said she was trying to understand the terms "yellow dog" and "blue dog".
I made the mistake of trying to figure it out:
"Yellow dog" comes from the expression, "I'd rather vote for a yellow dog than for a Republican.". "Blue dog" is a newer term referring to Democrats who tend to vote like Republicans, who live in "red" - Republican - states (our election return maps show Democrats in blue, Republicans in red).
I sometimes post (as 'zabouti') on bluenc.com, a liberal blog that really dislikes blue dog Dems. Go figure!