A controversial measure that would provide $3 billion in state bond money to pay for human embryonic stem cell research holds a narrow margin of support among voters, according to a new Field Poll.

Proposition 71, designed to get around the Bush administration's funding limits on such research, would authorize the state to sell bonds to provide annual payments of about $300 million to scientists and companies doing stem cell research.

The poll found that 46 percent of likely voters support the idea with 39 percent opposed and 15 percent undecided. The margin of error was 4.3 percentage points.

The numbers are a slight improvement from an August poll that found 45 percent supported the bond measure to 42 percent opposed.

Supporters of the proposition -- including a coalition of influential Democratic donors and several Silicon Valley tycoons -- have raised more than $12 million for the campaign and are running TV ads.

The Field Poll found that half of those surveyed said they had seen or heard something about the initiative -- an improvement since August of 10 percentage points when 40 percent had heard of the proposal.

While scientists and advocates say embryonic stem cell research could provide new treatments for many crippling diseases, critics argue that days-old embryos are destroyed in the process and thus call the practice immoral.

The poll, conducted among 549 voters during a six-day period ending Sept. 29, found that support for the measure fell along party lines: 57 percent of Democrats were in favor, while 55 percent of Republicans were opposed.

Of those who were in favor, 56 percent said they intend to vote for the measure because they believe more medical research is needed in this area. Another 40 percent said they wanted to see cures for troubling diseases.

Opponents of the initiative were split on their reasons: 26 percent said they were against the idea on religious or anti-abortion grounds; while 20 percent said they believed the measure would cost too much.