This group won 91 percent of Congressional races in which they endorsed a candidate

It was the NRA.

Via Breitbart:

With all the numbers tallied, out of 251 races, NRA-backed candidates won in 229, over 91 percent of Congressional races in which they endorsed a candidate.
According to the Washington Examiner, NRA spokesman Andrew Arulanandam explained that the Second Amendment group was able to win at this level because individual NRA members were motivated to vote for freedom. He said: “Our members came out in droves and voted for their rights and their freedom. This was one of the most successful election cycles in a decade.”
He listed some of the higher profile wins as the defeat of Senators Kay Hagan (D-NC) and Mark Pryor (D-Ark.), both of whom were replaced by NRA-endorsed candidates with an “A” rating on gun rights.
And while groups like Bloomberg’s Everytown for Gun Safety largely endorsed incumbents to pad their resume or piggy-backed on the NRA by endorsing the same candidate as the NRA in some races–Michigan Governor Rick Snyder (R) for instance–Arulanandam said the NRA threw its weight behind candidates in tough races when gun rights were on the line. One such race was in Virginia, where former pro-Second Amendment challenger Ed Gillespie (R) tried to unseat pro-gun control Senator Mark Warner (R-VA). Keep on reading...