SPRINGFIELD, Va. — When word surfaced in February that Senator Tom Coburn, Republican of Oklahoma, was plotting with Democrats on a bill to expand background checks for gun buyers, Larry Pratt got really mad. Then, Mr. Pratt, the executive director of Gun Owners of America, got busy, mounting a lobbying blitz that helps explain why a bipartisan Senate deal on background checks remains elusive.

Within days, his staff, working from a nondescript space in a squat office building off the Beltway here — there isn’t even a nameplate on the door — was on the phone with supporters of his organization in Oklahoma. The group’s members were encouraged to inundate Mr. Coburn with e-mails and calls and to otherwise make it exceedingly clear to the senator that an enhanced background check law would not be tolerated.

It wasn’t long until Mr. Coburn, a gun rights advocate, had backed away from negotiations with Senator Charles E. Schumer of New York, the No. 3 Senate Democrat, securing at least a temporary victory for gun rights activists and frustrating advocates of new gun safety laws.

Once largely unknown, Gun Owners of America, with its war chest, membership and lobbying strength dwarfed by the National Rifle Association, is emerging as an influential force as a series of gun control measures heads to the Senate floor.