Tucked deep inside the 1,603-page federal spending measure is a provision that effectively ends the federal government's prohibition on medical marijuana and signals a major shift in drug policy. The bill's passage ... marks the first time Congress has approved nationally significant legislation backed by legalization advocates. It brings almost to a close two decades of tension between the states and Washington over medical use of marijuana. Under the provision, states where medical pot is legal would no longer need to worry about federal drug agents raiding retail operations. Agents would be prohibited from doing so. Congress for years had resisted calls to allow states to chart their own path on pot. The marijuana measure, which forbids the federal government from using any of its resources to impede state medical marijuana laws, was previously rejected half a dozen times. Even as Congress has shifted ground on medical marijuana, lawmakers remain uneasy about full legalization. Marijuana proponents nonetheless said they felt more confident than ever that Congress was drifting toward their point of view. Approval of the pot measure comes after the Obama administration directed federal prosecutors last year to stop enforcing drug laws that contradict state marijuana policies.