Some are worst than others as to being in the tank.
My CA party has a stupid rule that none can challenge a dem in good standing...no matter how bad they are. So they protect themselves. It is called the Willie Brown rule. It's unwritten by the way. But pretty much they are called machines for a reason.
The Chicago machine, for example, is infamous for the patronage and favors. This is why it is so hard for outsiders to challenge both parties. And why all the endorsements read like a list of who's who in the party. Politicians rely on a network of connections, and right now favors are getting called right and left.
This happens at all levels. Look at your local city hall and look at who scratches each other's back.