President Obama struck a professorial tone while discussing marijuana legalization in an interview with VICE News released yesterday. He lamented in broad terms the costs of locking up nonviolent drug offenders, and the disproportionate impact these policies have on minority communities.

Obama noted that in recent years these costs have drawn scrutiny from both liberal and conservative members of Congress, especially more libertarian-minded Republicans who are suspicious of government overreach and hawkish about budget issues.

But he stopped short of endorsing any recent Congressional bills seeking to ease federal marijuana laws, including a bipartisan Senate medical marijuana bill introduced last week or full legalization measures introduced in the House last month. Indeed, he didn't venture a personal opinion on the marijuana question at all.

Rather, he appeared content to let changes unfold at the state level. “At a certain point, if enough states end up decriminalizing, then Congress may then reschedule marijuana,” he said. States, in fact, have been leading the way on overhauling marijuana policy for decades now, starting in 1973 when Oregon became the first state to decriminalize marijuana possession in the wake of the federal Controlled Substances Act.

Today, four states plus the District have fully legalized the use of marijuana. 10 states have both decriminalized recreational marijuana and legalized medical marijuana, while an additional 13 states have done one or the other. All told, 27 states plus the District have relaxed their marijuana laws in some significant way. These states are home to 176 million Americans, or 55 percent of the population. If you add in the handful of states that have also allowed limited access to a specific type of marijuana-derived oil to treat certain ailments, that number becomes even higher.

As the gap between state and federal laws widens, pressure is likely to build on Congress to update a federal marijuana policy that hasn't changed since 1973. But change will come slowly at the federal level.

In the Senate, marijuana legislation would have to pass through the judiciary committee, currently chaired by Republican Chuck Grassley of Iowa. Grassley, who first ascended to national office in 1975, is not exactly what you'd call an advocate of criminal justice reform. He's already signaled his opposition to the Senate medical marijuana bill.

In the VICE interview, Obama positions himself above this fray, commenting as a disinterested observer. But  his word still carries great weight with the public. Throwing his support behind a bill or staking out a clear position on the issue could have a considerable effect on public opinion -- we saw this happen after the president's "evolution" on gay marriage, for instance.

Obama has little to lose from clarifying his stance on the marijuana question six years into his presidency, free from the constraint of a re-election battle. But on marijuana as with other social issues, Obama has been content to evolve.