Marijuana is the most popular illegal drug in the United States. Seventy years of criminal prohibition, "Just Say No" sloganeering and a federal drug war that now incarcerates 225,000 people a year have not diminished the availability or use of — or apparently the craving for — cannabis. And helping meet the demand is California, the nation's top grower. Marijuana production here results in an estimated $14 billion in sales, and its cultivation and distribution are now tightly woven into the state's economy. It is grown in homes, in backyards and even in national parks, including Yosemite.

Marijuana is popular, plentiful and lucrative, costing about $400 a pound to grow and yielding $6,000 a pound on the street. So it is perhaps inevitable that an attempt would be made to legalize it, as Proposition 19 — the Regulate, Control and Tax Cannabis Act of 2010 — purports to do. The act would authorize possession of one ounce of marijuana for personal consumption by people 21 and older, permit marijuana use in private residences or public places licensed for on-site consumption, and allow marijuana cultivation in private residences for personal use. It includes strong restrictions regarding the sale or use of marijuana to or around minors, and would permit city and county governments to regulate and tax it. Proponents of the proposition say it would bring public policy on marijuana into line with that on alcohol and cigarettes, both of which can be dangerous and deadly but are nonetheless legal. It is the criminalization of the drug that creates social problems, they say, including a violent drug war at the border, fueled in part by black-market profits, and millions of lives damaged by overzealous enforcement rather than by the drug itself.

The proposal has riveted national attention on California, as did Proposition 215, the Compassionate Use Act of 1996, which permitted the medicinal use of marijuana. Thirteen states have since adopted similar measures, and public approval for medical marijuana has increased significantly. Californians' independent streak and willingness to challenge federal authority have galvanized the national debate on legalization. The question now is whether we will do it again. Will we thumb our noses at Washington and blaze another new trail?

We should not.

Whether marijuana should be legal is a valid subject for discussion. Californians ought to welcome a debate about whether marijuana is any more dangerous than alcohol, whether legalization would or would not increase consumption, and whether crime would go down as a result of decriminalization. But Proposition 19 is so poorly thought out, badly crafted and replete with loopholes and contradictions that it offers an unstable platform on which to base such a weighty conversation.

Its flaws begin with the misleading title: Regulate, Control and Tax Act. Those are hefty words that suggest responsibility and order. But the proposition is in fact an invitation to chaos. It would permit each of California's 478 cities and 58 counties to create local regulations regarding the cultivation, possession and distribution of marijuana. In other words, the law could change hundreds of times from county to county. In Los Angeles County alone it could mean 88 different sets of regulations.

The proposition would have merited more serious consideration had it created a statewide regulatory framework for local governments, residents and businesses. But it still would have contained a fatal flaw: Californians cannot legalize marijuana. Regardless of how the vote goes on Nov. 2, under federal law marijuana will remain a Schedule I drug, whose use for any reason is proscribed by Congress. Sure, California could go it alone, but that would set up an inevitable conflict with the federal government that might not end well for the state. That experiment has been tried with medical marijuana, and the outcome has not inspired confidence. Up and down the state, an untold number of residents have faced federal prosecution for actions that were allowed under California law. It's true that the Obama administration has adopted a more tolerant position on state laws regulating medical marijuana, but there's no guarantee that the next administration will. Regardless, Obama's "drug czar," Gil Kerlikowske, has firmly stated that the administration will not condone marijuana's legalization for recreational purposes.

One reason given by Proposition 19 supporters for legalizing marijuana is that California is in dire fiscal straits, and taxing the cannabis crop could ultimately enrich state and local coffers by $1.4 billion a year. But again, critics say that argument is misleading. The act essentially requires local governments that choose to regulate and tax marijuana to establish new bureaucracies and departments, and much of the new revenue could be eaten up by the cumbersome process of permitting and licensing sales, consumption, cultivation and transportation.

Far from helping the state's economic outlook, Proposition 19 could cause substantial harm. For instance, it would put employers in a quandary by creating a protected class of on-the-job smokers, bestowing a legal right to use marijuana at work unless employers could actually prove that it would impair an employee's job performance. Employers would no longer have the right to screen for marijuana use or discipline a worker for being high. But common sense dictates that a drug-free environment is crucial at too many workplaces to name — schools, hospitals, emergency response and public safety agencies, among others.

The multiple conflicts with federal law, and the strong probability of confusing and contradictory municipal laws that would result from its passage, overwhelm the hypothetical benefits of Proposition 19.

This is the first of The Times' endorsements in the Nov. 2 election. Upon publication, they will be collected at latimes.com/opinion.