This spring, President Obama promised Mexican President Felipe Calderon that he would work to deter gunrunning south of the border. Behind the scenes, however, White House officials were putting the brakes on a proposal to require gun dealers to report bulk sales of the high-powered semiautomatic rifles favored by drug cartels.

Justice Department officials had asked for White House approval to require thousands of gun dealers along the border to report the purchases to the Bureau of Alcohol, Tobacco, Firearms and Explosives. ATF investigators expected to get leads on suspected arms traffickers.

Senior law enforcement sources said the proposal from the ATF was held up by the White House in early summer. The sources, who asked to be anonymous because they were discussing internal deliberations, said that the effort was shelved by then-White House chief of staff Rahm Emanuel, a veteran of battles with the gun lobby during the Clinton administration.

Ben LaBolt, a spokesman for Emanuel, who is running for mayor of Chicago, said Emanuel “did not stop the policy from being implemented.” Emanuel “has never taken a back seat to anyone when it comes to standing up to the NRA to keep guns out of the hands of dangerous criminals,” LaBolt said.

The plan — which officials knew would be strongly opposed by the National Rifle Association — was perceived as too volatile just before midterm elections, the sources said.

Last month, ATF Deputy Director Kenneth Melson asked the Justice Department to try again, law enforcement officials said.

On Friday, the ATF published the emergency proposal in the Federal Register. The proposal requires dealers to report to the bureau anytime they make two or more sales over a five-day period of semiautomatic rifles that have a caliber greater than .22 and a detachable magazine. It would be valid for six months.

Dealers have been required for decades to report the sales of multiple handguns to the ATF.

The gun lobby responded angrily to the emergency proposal.

“We remain opposed to further burdening America’s law-abiding firearms retailers with yet another onerous regulation that will do nothing to curb crime,” said Ted Novin, spokesman for the National Shooting Sports Foundation, a group that represents gun dealers and manufacturers. “Furthermore, multiple sales reporting of long guns will actually make it more difficult for licensed retailers to help law enforcement as traffickers modify their illegal schemes to circumvent the reporting requirement.”

In May, Obama assured Calderon that the administration would assist Mexico in curbing drug cartel violence, which has led to 30,000 deaths in Mexico.