Ever since California voters became the first in the nation to legalize medical marijuana in 1996, the state has faced unyielding opposition from the federal government, which insists it has the power to prohibit a drug it considers useless and dangerous.

That could all change with the next presidential election.

As the candidates prepare for a May 20 primary in Oregon, one of 12 states with a California-style law, Sen. Barack Obama of Illinois has become an increasingly firm advocate of ending federal intervention and letting states make their own rules when it comes to medical marijuana.

His Democratic rival, Sen. Hillary Rodham Clinton of New York, is less explicit, recently softening a pledge she made early in the campaign to halt federal raids in states with medical marijuana laws. But she has expressed none of the hostility that marked the response of her husband's administration to California's initiative, Proposition 215.

Sen. John McCain of Arizona, the Republican nominee-in-waiting, has gone back and forth on the issue - promising a medical marijuana patient at one campaign stop that seriously ill patients would never face arrest under a McCain administration, but ultimately endorsing the Bush administration's policy of federal raids and prosecutions.

Political battles over exempting medical patients from marijuana laws have been fought mostly in statehouses and at ballot boxes since 1996, when California voters repealed state criminal penalties for those who used the drug with their doctor's approval. But the federal government has played an important role in limiting the scope of those state laws, and their effectiveness over the next four years may be determined by the next president.

Bill Clinton's position

President Bill Clinton's administration opposed the California law from the start and won a court case allowing it to shut nonprofit organizations that supplied medical marijuana to members. Clinton's Justice Department also tried to punish California doctors who recommended marijuana to their patients by revoking their authority to prescribe any drugs, but federal courts backed the doctors.

The Bush administration has gone further, raiding medical marijuana growers and clinics, prosecuting suppliers under federal drug laws after winning a U.S. Supreme Court case, and pressuring commercial property owners to evict marijuana dispensaries by threatening legal action. The administration has also blocked a University of Massachusetts researcher's attempt to grow marijuana for studies of its medical properties.

Since 2001, federal prosecutors have won convictions in at least 28 California drug cases where defendants claimed they were supplying or using medical marijuana, according to the National Organization for the Reform of Marijuana Laws. Prosecutors have filed charges in 22 more cases, and authorities have raided 10 growers or dispensaries without filing charges, the group says.

The presidential candidates haven't discussed the issue in speeches or debates, but medical marijuana advocates regularly questioned them in Iowa and New Hampshire. The most sweeping changes were proposed by second-tier candidates - Democrats Mike Gravel, Dennis Kucinich and Chris Dodd and Republican Ron Paul called for repealing federal criminal penalties for marijuana - but of the remaining contenders, Obama has been the friendliest to advocates of medical marijuana.

At a November appearance in Audubon, Iowa, Obama recalled that his mother had died of cancer and said he saw no difference between doctor-prescribed morphine and marijuana as pain relievers. He said he would be open to allowing medical use of marijuana, if scientists and doctors concluded it was effective, but only under "strict guidelines," because he was "concerned about folks just kind of growing their own and saying it's for medicinal purposes."

Obama went a step further in an interview in March with the Mail Tribune newspaper in Medford, Ore. While still expressing qualms about patients growing their own supply or getting it from "mom-and-pop stores," he said it is "entirely appropriate" for a state to legalize the medical use of marijuana, "with the same controls as other drugs prescribed by doctors."

In response to recent questions from The Chronicle about medical marijuana, Obama's campaign - the only one of the three contenders to reply - endorsed a hands-off federal policy.

"Voters and legislators in the states - from California to Nevada to Maine - have decided to provide their residents suffering from chronic diseases and serious illnesses like AIDS and cancer with medical marijuana to relieve their pain and suffering," said campaign spokesman Ben LaBolt.

"Obama supports the rights of states and local governments to make this choice - though he believes medical marijuana should be subject to (U.S. Food and Drug Administration) regulation like other drugs," LaBolt said. He said the FDA should consider how marijuana is regulated under federal law, while leaving states free to chart their own course.

Obama would end DEA raids

LaBolt also said Obama would end U.S. Drug Enforcement Administration raids on medical marijuana suppliers in states with their own laws.

Those raids have been the focus of Hillary Clinton's comments on the issue. At a July campaign event in Manchester, N.H., she told a medical marijuana advocate that she would end the federal raids, according to Granite Staters for Medical Marijuana, which recorded the exchange.

But the candidate was less absolute in a more recent interview with the Willamette Week newspaper in Hillsboro, Ore.

"I don't think it's a good use of federal law enforcement resources to be going after people who are supplying marijuana for medicinal purposes," Clinton said in the April 5 interview. But when asked whether she would stop the raids, she replied, "What we should do is prioritize what the DEA should be doing, and that would not be a high priority. There's a lot of other, more important work that needs to be done."

Clinton has also said she opposes repealing criminal penalties for marijuana, but told advocates in October that the government should conduct more research "into what, if any, medical benefits it has."

McCain has taken a variety of positions, according to comments recorded by medical marijuana advocates.

At an April 2007 campaign kickoff event, when asked if he would end federal raids, he said, "I would let states decide that issue." But less than two months later, he said he would not end the raids. Then, in November, he promised a man who described himself as a seriously ill marijuana patient that he would "do everything in my power" to make sure the man was never arrested for using the drug.

No policy paper

While maintaining that medical experts considered marijuana ineffectual and potentially dangerous, McCain promised at the same November event in New Hampshire to consult with experts and issue an "in-depth policy paper" on the topic within a few days. McCain's campaign has not responded to media inquiries, and marijuana advocates say the policy paper was never issued.

He was also asked during a November conference call whether the federal government should override the will of the people in states with medical marijuana laws. "Medical marijuana is not something that the, quote, people want," McCain replied.

Bruce Mirken, a spokesman for the advocacy group Marijuana Policy Project, said he remains hopeful that the federal climate will improve, no matter who becomes president.

"All it takes," he said, "is for the Justice Department to say, 'Leave these states alone.' "

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