People who use marijuana for medical purposes and those who distribute it to them should not face federal prosecution, provided they act according to state law, the Justice Department said Monday in a directive with far-reaching political and legal implications.

In a memorandum to federal prosecutors in the 14 states that make some allowance for the use of marijuana for medical purposes, the department said that it was committed to the “efficient and rational use” of its resources and that prosecuting patients and distributors who are in “clear and unambiguous compliance” with state laws did not meet that standard.

The new stance was hardly an enthusiastic embrace of medical marijuana, or the laws that allow it in some states, but signaled clearly that the administration thought there were more important priorities for prosecutors.

“It will not be a priority to use federal resources to prosecute patients with serious illnesses or their caregivers who are complying with state laws on medical marijuana,” Attorney General Eric H. Holder Jr. said in a statement accompanying the memo, “but we will not tolerate drug traffickers who hide behind claims of compliance with state law to mask activities that are clearly illegal.”

Emphasizing that it would continue to pursue those who use the concept of medical marijuana as a ruse, the department said, “Marijuana distribution in the United States remains the single largest source of revenue for the Mexican cartels,” and pursuing the makers and sellers of illegal drugs, including marijuana, will remain a “core priority.”

The new policy was tbe laszThe politics swirling around marijuana cross ideological lines. For instance, in effectively deferring to the states on some issues involving marijuana, the Obama administration is taking what could be seen as a states’ rights stance, more commonly associated with conservatives. That was a theme that echoed on many conservative and libertarian Internet sites in the wake of Monday’s announcement.

But one prominent conservative, Representative Lamar Smith of Texas, criticized the Justice Department’s position, saying it would weaken federal enforcement of drug laws.

“By directing federal law enforcement officers to ignore federal drug laws, the administration is tacitly condoning the use of marijuana in the United States,” said Mr. Smith, the senior Republican on the House Judiciary Committee. “If we want to win the war on drugs, federal prosecutors have a responsibility to investigate and prosecute all medical marijuana dispensaries and not just those that are merely fronts for illegal marijuana distribution.”

Polls have shown for years that there is widespread public support for making marijuana available to relieve the suffering of people who are very ill. But repeated efforts in Congress to block federal prosecution of medical marijuana have fallen short, and the new policy was a sharp departure from that of the Bush administration, in which the Drug Enforcement Administration raided medical marijuana distributors even if the distributors appeared to be complying with state laws.

The new policy, which reflects positions that Mr. Obama took as a presidential candidate and that Mr. Holder laid out in March, came in a memo from David W. Ogden, the deputy attorney general, to the United States attorneys in the affected states, most notably California.

The White House sought to turn aside any impression that Mr. Obama would like other states to follow the example of the 14 that make some allowance for medical marijuana.

“I’m not going to get into what states should do,” said the president’s chief spokesman, Robert Gibbs.

Mr. Gibbs said the memo to federal prosecutors “simply adds guidelines to a decision that Attorney General Holder talked about in mid-March and has been administration policy since the beginning of this administration in January.”

The guidelines give specific examples of conduct that would causes prosecutors to look at a case involving marijuana even if a user or distributor said it was for medical use. The examples include unlawful possession or use of a firearm, sales to minors and evidence of money laundering activity.

Graham Boyd, director of the Drug Law Reform Project at the American Civil Liberties Union, called the Justice Department’s move “an enormous step in the right direction and, no doubt, a great relief to the thousands of Americans who benefit from the medical use of marijuana.”

Mr. Boyd predicted that states and cities “will have a strong incentive to create regulated, safe and sensible means of getting marijuana to patients who need it.”

The new policy follows a series of changes, including the appointment of Richard Gil Kerlikowske, a former police chief of Seattle, to be Mr. Obama’s top drug policy adviser.

Medical marijuana thrived in Seattle on Mr. Kerlikowske’s watch, and advocates of more liberal marijuana laws hoped that his appointment to the office, which he assumed in May, signaled the administration’s willingness to decriminalize medical marijuana.

Some federal law enforcement officials are opposed to the administration’s position.

Privately, some federal law enforcement officials complained that medical marijuana and marijuana being smuggled in from Mexico are one and the same, and that the Obama administration has backed away from necessary enforcement of drug laws. Agents from the D.E.A. often work alongside local police officers.

As Mr. Ogden’s memo was being made public, the Web site of the Drug Enforcement Administration outlined its position on medical marijuana: “Smoked marijuana has not withstood the rigors of science — it is not medicine and it is not safe. D.E.A. targets criminals engaged in cultivation and trafficking, not the sick and dying.”

Advocates of medical marijuana say it can reduce chronic pain, nausea and additional symptoms associated with cancer and other serious illnesses. In 1996, California became the first state to make it legal to sell marijuana to people with doctors’ prescriptions. The other states that allow some use of marijuana for medical purposes are Alaska, Colorado, Hawaii, Maine, Maryland, Michigan, Montana, Nevada, New Mexico, Oregon, Rhode Island, Vermont and Washington. By David Stout & Solomon Moore, The New York Times.