Survey results released on Thursday showed 50 percent of likely voters backing the June 5 ballot measure and 42 opposed and 8 percent undecided.

Field Poll Director Mark DiCamillo said the findings suggest Proposition 29 could see support shrink in coming days, which would mirror two prior ballot battles over increasing tobacco taxes in the most populous U.S. state.

"With an eight-point lead it has a better than even chance of passage but it's going to be a close election," DiCamillo said.

The Field Poll survey's findings are in line with results of a recent survey by the Public Policy Institute of California. It found 53 percent of likely voters saying they will vote for the measure, 42 percent saying they will vote against it and 5 percent undecided.

In March, a survey by the institute found 67 percent of likely voters in support of Proposition 29, suggesting an advertising blitz by the tobacco industry has successfully raised concerns about the measure among voters.

They overwhelmingly support the idea of raising tobacco taxes but advertising has raised doubts about who would oversee revenue raised by Proposition 29, how it would be spent and whether it would stay in California.

The measure would raise California's tax on a pack of cigarettes to $1.87 to fund medical research into tobacco-related illnesses and programs to prevent tobacco use.

The money would be overseen by a nine-member committee that includes three University of California chancellors, three cancer research institute directors, one physician affiliated with an academic medical center and two members of disease advocacy groups that focus on tobacco-related illnesses.

The Field Poll survey's findings likewise suggest heavy industry spending on television and other advertising has eroded support for the measure.

"The current Field Poll shows that voters who had already sent in a mail ballot or were planning to do so before Election Day support Prop. 29 by a ten-point margin (51 percent to 41 percent). However, support narrows to just five points among those intending to vote at their voting precinct on June 5," the survey's report said.

"This suggests that the weight of campaign advertising and late decision-making is narrowing the yes side advantage and could have additional effects before all the votes are cast," the report said.

DiCamillo noted that California voters in 2006 rejected a ballot measure proposing a $2.60 tax increase to a pack of cigarettes by 52 percent to 48 percent - after the measure held substantial early leads in polls.

California voters last approved a measure to increase the state's tobacco tax in 1998, and only narrowly by 51 percent to 49 percent. A Field Poll survey a week ahead of that election found the measure with a nine-point lead, down from a 22-point lead in earlier polls.

The Field Poll conducted its latest survey from May 21 through May 29. Its findings are based on responses from 608 likely voters in the June election who were interviewed in English and Spanish over landline telephones or mobile phones.

The survey's maximum sampling error estimates for results based on its sample of likely voters is plus or minus 4.1 percentage points at the 95 percent confidence level.

(Reporting By Jim Christie; Editing by Chizu Nomiyama)

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