"Yellow dog" Democrats are non-thinkers; they vote the Democrat ticket no matter who's on it--they would vote for a yellow dog. They brag about it.
I see that's how it is being used, but I always remembered it from long ago as someone from Alabama who said it as a derogatory accusation to his fellow Dems, when he wanted to break from the ranks and support a Pubbie.
He accused them of it - and we who were Pubbies had championed his cause (this was long before my time that it actually happened), by considering it a derogatory term. Those who actually are that type of Dem seem to think it's a compliment - maybe like broken glass Republicans.
I have crawled over broken glass to vote, symbolically speaking, but not just because I was going to vote straight-ticket Republican. It was just because I was going to vote, period.