MP's can advocate a position in the party room, but if they lose the vote they're bound by the majority decision. There's a lack of sympathy for Payman within the ALP because she's said to have not even argued a position in the party room. It's probably an arcane principle in that the Libs don't have it, but their members don't routinely cross the floor. They simply have their cards marked never to be promoted and serve out their time either on the backbench and/or undermining the leader.
