MEXICO CITY — President Felipe Calderón, who has made fighting drug traffickers the centerpiece of his administration, proposed legislation on Thursday that would decriminalize the possession of small quantities of cocaine and other drugs for addicts who agreed to undergo treatment.

Mr. Calderón said that the proposal was intended to attack the growing problem of drug addiction in Mexico. Still, it will probably be controversial both at home and abroad. A similar measure two years ago provoked strong opposition from the United States and was eventually dropped.

A recent government survey found that the number of drug addicts in Mexico had almost doubled in the past six years to 307,000, while the number of those who had tried drugs rose to 4.5 million from 3.5 million.

Drugs used to flow through Mexico to the United States, and they still do, but an increasing amount of those narcotics now stays in Mexico to feed the habits of domestic consumers.

Under Mr. Calderón’s proposal, Mexican authorities would not prosecute people found to be carrying small amounts of drugs if they declared they were addicts and submitted to a treatment program.

Those who are not addicts could avoid prosecution by entering a prevention program. Fines could be imposed for those who declined to enter such programs.

The new legislation caps the quantities that would not be subject to prosecution at 50 milligrams of heroin, 2 grams of marijuana, 500 milligrams of cocaine and 40 milligrams of methamphetamine.

The Mexican attorney general’s office has said that it is so overwhelmed with prosecuting organized crime that it cannot handle the large number of small-time drug cases.

The measure is reminiscent of a proposal that passed the Mexican Congress two years ago but never took effect. It decriminalized possession of small amounts of drugs for people who could convince a judge that they were addicts.

That law, which did not require treatment for those found with drugs, provoked an uproar among United States officials, some of whom raised the image of Americans going to Mexico to enjoy legal drugs.

Under intense lobbying from the United States, Vicente Fox, the president at the time, asked Congress to amend the law and the measure was dropped.

Responding to Mr. Calderón’s plan, American officials said Thursday that United States policy opposed the legalization of even small amounts of drugs. “It rewards the drug traffickers and doesn’t make children’s lives safer,â€