I had no idea they were dumb enough to only be for one party. (should have actually read it I guess) Even scientists can be idiots.
Yeah, their stated rationale is incredibly counterproductive even if sincere and it's probably just supercilious tribalism.
I've no love for the NRA, but one thing that is impressive about them (so I've heard, Americans please correct me if I'm wrong), is how they focus just on their issue. If a Democrat running for State Senate is an inch more pro-gun than the Republican, they'll get the endorsement.
As Evangelicals, or blacks, or union members have learned at various times and places, you'll be much more effective in getting what you want if the party you support knows you've got one foot out the door, than if you mindlessly back one party.