Such is the power of the NRA. With annual revenue of about $250 million, the group has for four decades been the strongest force shaping the nation's gun laws.

The fate of the Mexican gunrunning rule is only the most recent example of how the gun lobby has consistently outmaneuvered and hemmed in ATF, using political muscle to intimidate lawmakers and erect barriers to tougher gun laws. Over nearly four decades, the NRA has wielded remarkable influence over Congress, persuading lawmakers to curb ATF's budget and mission and to call agency officials to account at oversight hearings. The source of the NRA's power is its focus on one issue and its ability to get pro-gun candidates elected.

The result is that a president such as Obama, whose campaign platform called for tougher gun laws, finds his freedom of action circumscribed. The issue has bedeviled Democrats for years, especially after defeats in the 1994 midterms and the 2000 presidential election, in which Al Gore lost his home state of Tennessee.

"That was the shift of the tectonic plate for the Democrats on the gun issue," said James Cavanaugh, former ATF special agent in charge in Nashville. "The thing that really, really, really scared the Democrats was Al Gore losing his home state, and the reason was the gun issue. They all know it."