Los Angeles ? After taking a serious look at legalizing marijuana, California’s voters on Tuesday rejected Proposition 19, which would have made the state the first to allow the drug to be sold and taxed.

According to a state exit pool and early election returns, the measure drew strong support from voters under 25 years old, as the campaign had hoped, but they did not turn out in unusually high numbers. The initiative also failed to win over the moderate voters who make up the state’s decisive swing vote.

The San Francisco Bay Area was the only region to tilt toward the measure, but just slightly. In Los Angeles County, where a quarter of the state’s voters live, the initiative lost.

Despite a potential double-digit loss, marijuana-legalization advocates said the proposition had transformed talk about legal pot from late-night punch line into a serious policy matter.

“This has been a watershed moment,” said Stephen Gutwillig, the California director for the Drug Policy Alliance, which waged an extensive ad campaign or the measure. “Even in defeat, Proposition 19 has moved marijuana legalization in to the mainstream of American politics.”

Tuesday’s vote was jut the first round, say legalization advocates, who are aiming measures at the 2012 ballot in Washington, Oregon, Colorado and very likely California. But it’s also the second time in two years that California voters have rejected an initiative to soften penalties for drug crimes.