This is fascinating! I have been a union man most of my working life, and have not come across this expression. That in itself is not, prhaps, surprising as it is obviously a US expression, and the left over here do not follow the American lead, if they can help it. But what is surprising is that we do not, so far as I am aware, have an equivalent expression.
It is not as if British Unions don't have paid officials - they have those in abundance - or that they don't hve welfare schemes for their members which can be (and occasionally are) abused by officials. But we don't have a word for it. Very strange.