By the time the 21st Amendment ended national alcohol prohibition in December 1933, more than a dozen states had already opted out. Maryland never passed its own version of the Volstead Act, while New York repealed its alcohol prohibition law in 1923. Eleven other states eliminated their statutes by referendum in November 1932.

We could see the beginning of a similar rebellion against marijuana prohibition this year as voters in three states—Washington, Colorado, and Oregon—decide whether to legalize the drug's production and sale for recreational use. If any of these ballot initiatives pass, it might be the most consequential election result this fall, forcing both major parties to confront an unjust, irrational policy that Americans increasingly oppose.

With six weeks to go before Election Day, Oregon's Measure 80, which would establish a commission charged with licensing growers and selling marijuana through state-run stores, seems to be in trouble. In a SurveyUSA poll this month, only 37 percent of respondents said they planned to vote yes, while 41 percent were opposed and 22 percent were undecided.

But the other two initiatives are polling strongly. According to a SurveyUSA poll conducted two weeks ago, 57 percent of Washington voters favor Initiative 502, which would authorize private pot stores regulated by the state liquor commission; only 34 percent were opposed. A SurveyUSA poll completed on September 12 found that 51 percent of Colorado voters support Amendment 64, which would allow home cultivation of up to six plants and create a licensing system for growers and retailers; 40 percent were opposed.

Neither of these measures is a sure thing by any means. California's Proposition 19, a marijuana legalization measure that was ultimately supported by 47 percent of voters in November 2010, polled above 50 percent in several surveys. But while the SurveyUSA approval number for Proposition 19 peaked at 56 percent in April 2010, dropping to 47 percent by September, support for the Washington and Colorado initiatives appears to be growing.

In the Colorado survey, supporters outnumbered opponents in every age group except respondents 65 or older, and Amendment 64 was ahead by 30 percentage points among respondents younger than 35. These generational differences are clear in national survey data as well. In a 2011 Gallup poll, 62 percent of 18-to-29-year-olds supported marijuana legalization, compared to 31 percent of respondents in the 65-and-up group.

Overall support for legalization in the Gallup survey was the highest it has ever been: 50 percent, compared to 12 percent in 1969 and the mid-to-high 20s during the Carter administration, which was later viewed as an especially pot-tolerant period. A May Rasmussen survey put current support even higher: 56 of respondents said marijuana should be treated like alcohol, making pot legalization more popular than Barack Obama or Mitt Romney.

Rising support for legalizing marijuana parallels increasing experience with the drug. The federal government's survey data indicate that most American adults born after World War II have tried pot, an experience especially common among people now in their 20s, 30s, and 40s.

That does not mean all these people are current marijuana consumers, eager for the lower prices, convenience, quality, and variety promised by a legal market. But they, along with their friends and relatives, have had enough direct and indirect experience with cannabis to decide that prohibition costs more than it's worth.

As The Seattle Times observed in a recent editorial endorsing Initiative 502, "The question for voters is not whether marijuana is good. It is whether prohibition is good." The voices rejecting prohibition in Washington and Colorado include city council members, state legislators, former U.S. attorneys, clergymen, retired cops, and two national police organizations—a hard group to dismiss as a bunch of silly potheads, which is President Obama's usual approach to the issue.

If voters approve marijuana legalization in one or more states this November, that contemptuous attitude will no longer be tenable, no matter who wins the presidential election.