Many lawmakers and gun safety advocates believe Gun Owners of America’s rising profile and heavy membership drive has led the N.R.A. to take a more aggressive stance against measures it once supported, like an expansion of background checks to include private gun sales. (In 1999, Wayne LaPierre, the chief executive of the N.R.A., said there should be “no loopholes anywhere, for anyone” on gun sale background checks.)

Part of the group’s mission, Mr. Pratt said, is to stay on top of the N.R.A. “when we don’t think they’ve gone far enough.”

“The N.R.A. is essentially a political party,” said Mark Glaze, the director of Mayors Against Illegal Guns. “The big majority is centrist, and the base is ideological. And they’re pulled to the right in a competition for the base with Larry Pratt, and that’s marginalizing them.”

Gun Owners of America’s central mission now is to prevent the passage of several bills about to hit the Senate floor. It is also lobbying for a House bill that would eliminate gun-free school zones.