At issue is whether Congress should loosen restrictions on local law enforcement agencies' ability to gain access to gun-purchasing data they want to trace the movement of illegal guns around the nation.
The restrictions on such "trace data" began almost four years ago when Rep. Todd Tiahrt, R-Kan., succeeded in limiting the Bureau of Alcohol, Tobacco and Firearms, or ATF, from publicly revealing information from its gun trace database.
On Thursday, the battle shifts to the Senate Appropriations Committee, where Sens. Barbara Mikulski, D-Md., and Frank Lautenberg, D-N.J., are among those trying to repeal or weaken the gun data restrictions when the panel acts on the ATF's budget. Pro-gun rights stalwarts including Richard Shelby, R-Ala., and Ben Nelson, D-Neb., are pushing back hard and seem poised for victory.
The NRA says the data-sharing restrictions protect gun owners' privacy, but mayors around the country such as New York City Mayor Michael Bloomberg contend they hamper law enforcement authorities' ability to trace illegal guns and arrest weapons traffickers.
The mayors say gun trace data helps local police departments figure out where illegal guns are coming from, who buys them and how they get trafficked into their communities. Most guns used in crimes are sold by a small number of rogue gun dealers.
"The fight is between the nation's mayors and law enforcement leaders on one side, and the gun lobby on the other," said Paul Helmke, president of the Brady Campaign to Prevent Gun Violence. "The individuals who benefit most from the Tiahrt restrictions are corrupt gun dealers and illegal gun traffickers."
Bloomberg, who recently left the GOP amid speculation he may run for president, has sued numerous out-of-state gun dealers in an attempt to reduce the flow of illegal guns into New York. The NRA-backed restrictions block cities from getting ATF data for such lawsuits.
The NRA says it would limit the release of the information to criminal investigators, and keep the information away from antigun activists, headline-hungry politicians and opportunistic trial lawyers.
Gun control advocates have had little success on Capitol Hill since a Democratic-controlled Congress muscled through an assault weapons ban in 1994. Many Democrats credited the ban for losses in rural seats as the party took a drubbing at the polls that year.
This year, Democrats owe their narrow majorities in the House and Senate to freshmen from rural and Republican-leaning areas. Such pro-gun members include Sens. Jim Webb, D-Va., and Jon Tester, D-Mont., and numerous moderate "Blue Dog" Democrats elected to the House last year.
The difficult route to overturn the gun trace data restrictions contrasts sharply with the smooth path through the House of legislation aimed at correcting flaws in the national gun background check system that allowed a Virginia Tech student who killed 32 others to buy guns despite his diagnosed mental health problems.
That legislation would require states to automate their lists of convicted criminals and the mentally ill, who are prohibited under a 1968 law from buying firearms, and report those lists to the FBI's National Instant Criminal Background Check System
The difference is that the NRA endorsed the background check improvements, boosting its chances of becoming the first major national gun control law in more than a decade.
The House has yet to debate companion legislation. But West Virginia Democrat Alan Mollohan, chairman of the appropriations panel funding the ATF budget, has announced plans to stick with the current restrictions on ATF gun trace information.