The National Rifle Association appears to be a very powerful lobby in the United States which seems to be very successful at preventing anti-gun legislation. Nevertheless, it would appear to be quite feasible for there to be a competing lobby operating at the level of the NRA to lobby Congress for more gun control legislation.
A search for "anti NRA" reveals there are organizations opposed to NRA-style policies, but they aren't anywhere near as well-known or powerful. Why is that? Is it really that far-fetched that 5 million Americans would support anti-NRA-style lobbying with their wallets, to offset the 5 million members of the NRA?
What has prevented the growth of an anti-gun lobby as powerful as the pro-gun NRA?