LOS ANGELES – It seemed an easy sell in California: The state that gave us medical marijuana would allow pot for recreation.

Then came the ads, newspaper editorials and politicians, warning of a world where stoned drivers would crash school buses, nurses would show up at work high and employers would be helpless to fire drug-addled workers.

A day after voters rejected Proposition 19, marijuana advocates wondered how they failed in trendsetting, liberal California.

But activists vowed Wednesday to push on in California, as well as in states that rejected other pot measures Tuesday.

“Social change doesn’t happen overnight,” said Paul Armentano, deputy director of the National Organization for Repeal of Marijuana Laws.

In South Dakota, voters rejected for the second time a medical marijuana measure — a step first taken by California in 1996 and by 13 other states since. Oregon voters refused to expand their medical marijuana program to create a network of state-licensed nonprofit dispensaries.

California’s initiative, which would have allowed adults age 21 and older to possess and grow small amounts of marijuana, failed 54 to 46 percent.

“There is a sense of people wanting to move into a new policy but still being wary of what that change might mean,” said Ethan Nadelmann, director of the pro-legalization Drug Policy Project.

Roger Salazar, a spokesman for the campaign to defeat Prop 19, agreed that misgivings about possible social problems from increased marijuana use helped seal the measure’s fate.

But he also blamed backers for leaving it up to local governments to set sales regulations. He also faulted them for promoting the measure as a revenue windfall for the state.

“The risks of legalizing something as important as marijuana were far greater than the potential benefits, and the benefits were far from guaranteed,” Salazar said.

Just a month before the election, Gov. Arnold Schwarzenegger signed legislation that made possession of up to an ounce of marijuana the equivalent of a traffic ticket, subject to no more than a $100 fine.

Prop 19 supporters said they believed he signed the bill to undercut any sense of urgency around marijuana legalization.

Abbey Kaufman and Matt McDonald toked up “a few blunts” during the Giants World Series celebration in front of San Francisco City Hall despite a strong police presence.

The 20-year-olds said they voted yes but weren’t disappointed that Prop 19 failed.

“Right now, you can smoke as much pot as you want on the streets of San Francisco,” Kaufman said. “If it had passed, marijuana would have been treated like booze and there would be a big crackdown on public smoking.”