Something that is "bread-and-butter" is very basic, or simple, or the key part of something. Thus a "liberal bread-and-butter issue" would be something on which all liberals agreed, or something that was taken to be infallibly part of their ideology. Therefore it can be used to refer to a person's preferred activity; you often would hear football commentators saying "Free kicks are John Terry's bread and butter".

bread-and-butter = very basic Bread and butter is a very basic food in England. That's where the expression comes from.

bread and butter issues= housing, health, sanitation