It's been attempted before, but that kind of thing is really hard to legislate effectively. At what point is something tangentially related? True, gun control and abortion bills should probably be kept separate, but what about closing the gun show loophole and also requiring conceal carry permits to have interviews? They're both about guns, but not the same issue relating to guns. It isn't really feasible, and even if it were, both parties like to pull the amend a bill to hell trick.