LOS ANGELES >> California voters declined to make their trendsetting state the nation’s first to legalize marijuana use and sales, heeding warnings of legal chaos and that pot smokers would get behind the wheel and show up to work while high.

The legalization effort was losing by nine percentage points with more than two-thirds of precincts reporting. Backers showed support for the measure by gathering outside the campaign’s headquarters to watch returns come in — some of them lighting up joints to mark the occasion.

Supporters of Proposition 19 blamed Tuesday’s outcome on the conservative leanings of older voters who participate in midterm elections. They also acknowledged that young voters had not turned out in sufficient numbers to secure victory, but said they were ready to try again in two years.

“It’s still a historic moment in this very long struggle to end decades of failed marijuana prohibition,” said Stephen Gutwillig, California director for the Drug Policy Project. “Unquestionably, because of Proposition 19, marijuana legalization initiatives will be on the ballot in a number of states in 2012, and California is in the mix.”

Tim Rosales, who managed the No on 19 campaign, scoffed at that attitude from the losing side.

“If they think they are going to be back in two years, they must be smoking something,” he said. “This is a state that just bucked the national trend and went pretty hard on the Democratic side, but yet in the same vote opposed Prop 19. I think that says volumes as far as where California voters are on this issue.”

The campaign pitted the state’s political and law enforcement establishment against determined activists. Images of marijuana leaves and smashed-up cars and school buses appeared in dueling ads during the campaign.

In a sign of what a tough sell it was, an exit poll conducted for The Associated Press showed opposition cutting across gender and racial lines, as well as income and education levels.

The ballot measure lost in the state’s vaunted marijuana-growing region known as the “Emerald Triangle” of Humboldt, Mendocino and Trinity counties. Many in the region feared the system they have created would be taken over by corporations or lose its purpose.

Proponents pitched it as a sensible, though unprecedented, experiment that would provide tax revenue for the cash-strapped state, dent the drug-related violence in Mexico by causing pot prices to plummet, and reduce marijuana arrests that they say disproportionately target minority youth.

In the weeks leading to the election, federal officials said they planned to continue enforcing laws making marijuana possession and sales illegal and were considering suing to overturn the California initiative if voters approved it.

“Today, Californians recognized that legalizing marijuana will not make our citizens healthier, solve California’s budget crisis, or reduce drug related violence in Mexico,” White House Drug Policy Director Gil Kerlikowske said.

Voters in three other states cast ballots on medical marijuana-related measures.

In South Dakota, voters rejected for the second time a measure to legalize marijuana for medical use — a step taken by California in 1996 and 13 other states since. Oregon voters refused to expand their state’s medical marijuana program to create a network of state-licensed nonprofit dispensaries where patients could have purchased the drug.

A medical marijuana measure on Arizona’s ballot was too close to call early Wednesday.

California’s marijuana proposal would have allowed adults 21 and over to possess up to an ounce of pot, consume it in nonpublic places as long as no children were present, and grow it in small private plots.

It also would have authorized local governments to permit commercial pot cultivation, as well as the sale and use of marijuana at licensed establishments.