SACRAMENTO, Calif. (AP) -- Two weeks after California elections, a closely watched effort to impose a new tax on tobacco in the nation's most populous state remains too close to call.

With 400,000 ballots outstanding as of Tuesday, the measure that would add a $1-a-pack cigarette tax is trailing by 17,500 votes, according to data compiled by the secretary of state.

Through a barrage of campaign ads, tobacco companies were able to cut support for the tax plan spearheaded by champion cyclist Lance Armstrong. Backing for the measure dwindled from a two-thirds majority in March down to a dead heat on Election Day.

Opponents raised $47 million to fight the proposal, dramatically outspending supporters, who raised $12 million.

Since the June 5 voting, Proposition 29 has seemed headed for defeat by razor-thin margins, generally trailing by less than a percentage point.

In election night returns, the proposal, which would in part fund cancer research, was losing by tens of thousands of votes, prompting many to assume it was dead. But supporters have refused to concede defeat.

"Last week, someone was thinking about having a press conference and conceding, and everybody else said, `Are you out of your mind?'" said Stan Glantz of the University of California, San Francisco's Center for Tobacco Control Research and Education.

Glantz has been running statistical analyses of the returns since the polls closed and said the chances of a reversal are "unlikely but not impossible."

"At this point, everybody's just biting their fingernails," he said.

Tobacco tax opponents are also watching returns, but with a more optimistic eye.

"We're not expecting any huge swings, and we are anticipating that our lead will hold," said Beth Miller, spokeswoman for the No on 29 campaign.

In order for the tax to squeak through, 54 percent of the remaining ballots would have to favor the measure, said Contra Costa Registrar of Voters Steve Weir.

Weir added that if the margin continued to tighten, it was conceivable that California would hold its first statewide re-count for a ballot measure.

"For there to be less than a half a percent spread, that's very, very unusual," he said.

Observers around the nation have also been watching the contest. Smoking foes say that Big Tobacco's success in branding the tax as a government boondoggle could reverberate in other states.

Election officials have until July 6 to report final results. Another California tobacco tax measure lost by a thin margin in 2006.

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