The NRA opposed it because there was an arbitrary process where virtually anyone could enter a person on the terrorist watch list, which is kept secret, and no legal means to get your name off the list once it was put on. In otherwords, liberal hacks like Lois Lerner could add the names of conservatives to the list and it would be impossible to get them removed.
Exactly. Anyone an enemy of the current regime could be placed on there for any reason, they don’t have to tell you, and you’d never be able to fight it.