More could be said, but there was a good discussion 4 years back of the history of guns as a wedge issue here , with u/GorrestFump and u/tablinum.

I don't think there's much contention about the 1968 Gun Control Act being a big part, but Frank Smyth's recent book ( The NRA:The Unauthorized History) points at the NRA 1977 Cincinnati Convention as the real important shift, because immediately afterwards it was able to raise much more money, grow in size, and get more vigorous support from its membership as a political organization than it ever had had as a shooting sports organization.