Major policy changes often happen as a result of a sudden shift that is, in fact, not so sudden at all. Public attitudes and behavior steadily change over time, but a political system whose practitioners have made up their minds on a topic years ago, before that change became apparent, are typically unwilling to accept the new reality. Until something changes – a new generation of leaders takes power, a financial crisis causes people to become more open to new ideas. Or perhaps it’s just as simple as an idea whose time has come, an idea whose wisdom can no longer be denied.

We’re at such a turning point with marijuana. One of the state’s main cash crops, the economic base of many small towns in the North Coast (and of a growing but hard to track number of metropolitan households), marijuana is already widely available in California, whether on the black market or at a quasi-legal dispensary. As more and more Californians are comfortable with the use of marijuana, even if they do not partake of it themselves, the decades-old drug war has become seen as more and more absurd when it comes to marijuana.

When an April Field Poll found 56% of Californians back marijuana legalization, it became only a matter of time before the topic became a fully mainstream subject, deemed appropriate for “serious” conversation at everything from public policy summits to the dinner table.

And so this week California is witnessing a fundamental shift in marijuana policy, where for perhaps the first time it really is a question of “when,” and not “if,” the sale and use of marijuana will become legal in California.

The biggest news comes from the federal government, where Attorney General Eric Holder has followed through on his early signals and announced the Justice Department will no longer prosecute people for using medical marijuana in accordance with their state’s laws. Holder is not yet embracing full legalization, of course. But this is a significant shift that recognizes states do have a right to innovate when it comes to drug policy. Whether the Obama Administration intends it or not, the new policy will be further evidence that a strict federal “War on Drugs” is no longer desirable or viable.

Here in California, more fundamental changes are under way. As a judge rules LA DA Steve Cooley’s attack on dispensaries to be invalid, the movement for full legalization is well under way. Tom Ammiano’s bill to legalize, regulate, and tax marijuana, AB 390, will get its first hearing in the Assembly next week.

Arnold Schwarzenegger, speaking at a bill signing ceremony in Merced yesterday, said he is “basically opposed” to legalization but believes it’s time to have a debate about the issue. In Arnold-speak that says he doesn’t see legalization as a political loser, even if he’s not quite willing to go there himself. His comments show that legalization has gone from being a sensible idea on the fringes of our political discourse to something we can debate as easily and naturally as, say, water policy.

Meanwhile, armed with the Field Poll results – as well as the recent Gallup Poll which found support for legalization was highest in the Western US, with moderates and independents nationwide about split on the matter, California activists are not waiting around for the legislature or the governor to act.

Instead they’re going directly to the ballot. TaxCannabis.org is the headquarters for the effort to put an initiative on the November 2010 ballot to treat marijuana much like alcohol. The initiative would legalize possession of up to one ounce for all adults over 21, and give local governments the ability to determine whether to more broadly legalize and tax marijuana themselves. It would essentially create a “local option” instead of a statewide free-for-all.

It’s not yet clear if they have the money or the volunteers to put this on the ballot. And the fact that local governments would be the ones implementing the policy, instead of a single statewide standard, might limit the savings in prison spending and the overall tax revenues created. But it’s a clear step forward for sensible drug policy, one whose time has clearly come.