I think you may have the outline of the story a little confused. If you go through old American Rifleman magazines ( the NRA's magazine) you can follow the organization's development from a relatively small group of target shooters and hunters in the 1920's, which became a bigger sportsman's organization in the '30's, successfully pushed for hunter safety and regulation in the 1940's, had big glossy photos of movie stars with their gun collections in the 1950's. But politics was never completely out of the picture: it was always somewhat conservative. When the country swung leftward and against the Vietnam War in the 60's, for example, as expected the NRA stuck to its long-standing support for the military. But with the Gun Control Act of 1968, following the Bobby Kennedy and MLK assassinations, it became illegal to send guns through the mail, and gun dealers had to be licensed. That got the NRA more directly focused on governmental issues.

The pivot in 1977 was , actually , a pivot from a recent pivot to go the other way. In 1976 the NRA leadership made big staff changes ( a lot of old employees were pretty summarily dismissed) and it began to talk of moving the headquarters from Washington DC to near Raton New Mexico and focusing on shooting sports...becoming something like the Isaac Walton League, cooling off on politics. The following convention in 1977, in Cincinnati, there was a revolt by the Life Members against this shift, lead by some gun-rights hardliners like Neal Knox ( regular members of the NRA do not get to vote, only Life Members). As a result of this, the NRA pretty much settled into being the political group it has been since. So, really, the answer to your question as to who funded the NRA in this: it was, really, the membership, which even more importantly has also been very active at rallying and agitating for gun rights, ready to dump any politician, no matter how qualified in any other respect, who does not agree with them.

Here's an excerpt from Michael Waldman's book, The Second Amendment, that adds more details to this story. Waldman generally thinks of the regulation of firearms as a Good Thing ( is not of the when-you-can-pry-it-from-my-dead-fingers school of gun rights), but he does get his basic facts straight ( though the excerpt may leave the impression that the NRA got Reagan elected President, which is a stretch). And, anyway, as this is a very hot topic ( and likely to get hotter) it is going to be hard to find anyone who can legitimately claim to be completely impartial about the history of the NRA and gun control in the US

EDIT: self disclosure. I am a gun owner and hunter. I am not a member or a supporter of the NRA or part of the Open Carry movement, do not look upon my deer rifle as the pillar of our democracy but am fond of it, since it puts venison in the freezer.