The latest Washington Poll, conducted from October 18 through October 31, suggests that state's marijuana legalization initiative probably will pass on Tuesday: 55.4 percent of likely voters favored Initiative 502, while just 37.6 percent were against it. That level of support is consistent with other polls taken this month and last. Colorado's marijuana legalization initiative, Amendment 64, may also pass. A Public Policy Polling survey conducted from October 23 through October 25 put support among likely voters at 53 percent, with 43 percent against. What happens if voters approve these measures?

Under Washington's I-502, penalties for possession of up to an ounce of marijuana by anyone 21 or older will be immediately repealed. Public consumption will be a Class 3 civil infraction, subject to a $50 fiine. Criminal penalties for selling marijuana paraphernalia also will be eliminated. The initiative creates a new legal standard for "driving under the influence of marijuana": five or more nanograms of THC per milliliter of blood for drivers 21 or older and any amount for drivers younger than 21. (These rules, intended to reassure voters worried about the traffic-safety implications of legalizing marijuana, have been controversial among drug policy activists because they may expose unimpaired pot smokers to the risk of license suspension and criminal penalties.) The Washington State Liquor Control Board has until December 1, 2013, to adopt regulations for marijuana growers, wholesalers, processors, and retailers. The state-licensed pot shops will be unobtrusive, with a single sign "identifying the retail outlet by the licensee's business or trade name," no cannabis products visible from the street, and no on-site consumption (which means Washington's legal, state-licensed pot shops will be more discreet than Amsterdam's technically illegal but tolerated cannabis cafés). The marijuana stores must be more than 1,000 feet from "any elementary or secondary school, playground, recreation center or facility, child care center, public park, public transit center, or library."

Unlike in Washington, pot smokers in Colorado will have an immediate state-legal source of marijuana. Amendment 64 legalizes not only possession of up to an ounce but also nonprofit transfers of that amount and home cultivation of up to six plants. As in Washington, "consumption that is conducted openly and publicly" would remain illegal. The Colorado Department of Revenue has until July 1, 2013, to adopt regulations for commercial marijuana producers and sellers. Amendment 64 allows local governments to ban pot stores within their jurisdictions, the same leeway they have under Colorado's law governing medical marijuana dispensaries. The initiative also requires the state legislature to "enact legislation governing the cultivation, processing, and sale of industrial hemp" by July 1, 2014.

Marijuana will still be prohibited under federal law. But contrary to an argument made by opponents of Proposition 19, the California legalization initiative that lost by five percentage points in 2010, that does not mean the Supremacy Clause makes these measures unconstitutional. As Jonathan Caulkins and three other drug policy scholars note in their new book Marijuana Legalization: What Everyone Needs to Know, "The Constitution does not allow the federal government either to order state governments to create any particular criminal law or to require state and local police to enforce federal criminal laws."

Even under national alcohol prohibition, which unlike the federal ban on marijuana was authorized by a constitutional amendment, states were free to go their own way. They could decline to pass their own versions of the Volstead Act (as Maryland did), repeal them (as a dozen states, including Colorado and Washington, did while the 18th Amendment was still in force), or simply refrain from prosecuting people under them (which was common in the wetter districts of the country). "The question is not whether a state could change its own laws," Caulkins et al. write. "Rather, the question is how the conflict with the continued federal prohibition would play out."

While the feds certainly can make trouble for any state that dares to legalize pot, there is a practical limit to what they can accomplish on their own. According to the FBI, there were about 750,000 marijuana arrests nationwide last year, the vast majority for possession. State and local police departments were responsible for something like 99 percent of those arrests. It simply is not feasible for the Drug Enforcement Administration (DEA)—which has about 5,500 special agents nationwide, compared to about 765,000 sworn personnel employed by state and local law enforcement agencies—to bust a significant percentage of people who grow pot for themselves and their friends (as Colorado's initiative allows), let alone people who possess it for recreational use.

The DEA can raid state-legal pot shops, as it has done with medical marijuana dispensaries, but the number of potential targets will be considerably larger once the market officially expands to include recreational users. The Justice Department can use asset forfeiture as an intimidation tactic against landlords and threaten banks that accept deposits from pot businesses with money laundering charges. The Internal Revenue Service can make life difficult for pot sellers by disallowing their business expenses (but not, thanks to a tax law wrinkle, their "cost of goods sold," which includes the cost of buying marijuana). The feds could even threaten state regulators with prosecution for handling marijuana or facilitating the trade, although that seems less likely, since it would provoke a direct confrontation with state officials. (Washington's initiative seeks to minimize this risk by assigning the task of testing marijuana for regulatory purposes to private, state-approved laboratories.) The one thing federal drug warriors cannot do, judging from their track record even when they have the full cooperation of state and local law enforcement agencies, is suppress the business entirely.