During the 2008 campaign, one of candidate Barack Obama’s best applause lines was a promise to restore respect for science when it came to federal policy making.

On Monday, President Obama kept a piece of that promise when his Department of Justice issued a directive ordering agency lawyers not to prosecute individuals who use or prescribe medical marijuana in states that have legalized the drug for that purpose.

“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, but we will not tolerate drug traffickers who hide behind claims of compliance with state law to mask activities that are clearly illegal,” explained Attorney General Eric Holder. “This balanced policy formalizes a sensible approach that the Department has been following since January: effectively focus our resources on serious drug traffickers while taking into account state and local laws.”

In the overall scheme of the drug-policy debate, this is a relatively small — and cautious — step.

But for medical-marijuana advocates, the administration’s formal embrace of a more responsible approach represents a major breakthrough.

“This is a huge victory for medical-marijuana patients,” says Steph Sherer, executive director of Americans for Safe Access, a medical-marijuana advocacy group. “This indicates that President Obama intends to keep his promise … and represents a significant departure from the policies of the Bush administration.”

The jury is in on medical marijuana and the evidence argues for removing barriers — federal, state and local — to its use by patients seeking relief from pain and nausea associated with cancer, AIDS and multiple sclerosis and other debilitating illnesses and conditions.

The Obama administration’s move respects that evidence. As such, it represents a clearer embrace of science with regard to drugs and drug policy by a White House than we have seen since the days when Jimmy Carter explored enlightened approaches.

As New York Congressman Maurice Hinchey said Monday, “Today, common sense won out over ideological stubbornness as our nation’s law enforcement agency formally adopted a new and well-balanced policy on medical marijuana use. Across the country, individual states have enacted laws that allow individuals who are sick and suffering to use medical marijuana with a doctor’s prescription only to have DOJ officials arrest and prosecute them anyway. This was a policy that was misguided and wrong from the start and I’m very pleased that the Obama administration’s Justice Department, under the leadership of Attorney General Holder, has put an end to it.”

Hinchey has for many years advocated for a shift in federal policies with regard to medical marijuana. Among other things, the New York Democrat has sought to amend Department of Justice appropriation bills in order to prevent the DOJ from using funds to prosecute individuals who use medical marijuana in compliance with state law.

Earlier this year, Hinchey secured House support for a requirement that the DOJ report to Congress about the administration’s position on medical marijuana.

That won’t be necessary now, as the DOJ order clarifies the issue — and sets a sounder policy regarding the lawful use of marijuana for medical purposes.

“Today,” says Hinchey, “those patients no longer have to worry that the medicine they’ve been legally using in their states will result in them being thrown in jail. Our Justice Department will now let these patients use medical marijuana in accordance with state law and federal prosecutors will instead focus their attention on more pressing legal matters that warrant their time and attention.”

The next step should be to codify the DOJ directive by passing Massachusetts Congressman Barney Franks’ Medical Marijuana Patient Protection Act (HR 2835), which would formally reschedule cannabis as a Schedule II drug and eliminate federal authority to prosecute medical marijuana patients or providers in states where it is legal for medical use.

Even before that happens, however, the DOJ directive opens the way for states to enact laws permitting the use of medical marijuana. Legislative bills and referendums have already been proposed in a number of states. Americans for Safe Access tracks the progress, which should accelerate considerably now that Obama’s Department of Justice has abandoned the backward and punitive policies of past administrations.