Support for Proposition 19, the state initiative that would legalize marijuana, is fading in the final days of the campaign, according to Field Poll results released today.

The same poll - the last one before Tuesday's vote - found continued opposition to Proposition 23, which would suspend the state's greenhouse gas reduction law, and support for Proposition 25, which would allow the Legislature to approve budgets with a simple majority instead of a two-thirds vote.

Of 1,501 registered voters who participated in the Field Poll, 49 percent said they oppose Prop. 19 and 42 percent said they are in favor of it - that's a complete reversal from the poll in September.

The Field Poll interviewed voters by phone between Oct. 14 and 26. The results come with a margin of error of 3.2 percentage points.

Voter opinions tend to shift during a campaign season, becoming more refined as the election draws closer, especially on issues that get a lot of media attention and last-minute campaign pushes, Field Poll officials said.

In fact, poll results on Prop. 19 have shifted continuously over the course of the campaign. In July, 48 percent of people polled were against Prop. 19. In September, 49 percent were in favor of it. Now, the opposition is leading again.

Prop. 19 opponents said they believe voter opinion is changing as people get more information about the specifics of the marijuana initiative.

"The more voters have focused on the language of the initiative itself, the more they've said, 'Wait a second, even if we're for the concept, maybe we'd better figure out a better way to do it,' " said Roger Salazar, spokesman for the No on Prop. 19 campaign.

But campaign leaders for Prop. 19 have suggested that polling voters about marijuana is tough to do, because people may be reluctant to tell a questioner that they support legalizing pot.

The campaign ran simultaneous polls this month - one in which voters were questioned by a live person, and one in which they were questioned by automated voices - and found that support was much higher when voters were talking to a computer.

Those differing results show that "many people are simply reluctant to admit their support on such a controversial matter," said Tom Angell, media director for Prop. 19. "We're confident that once people are in the privacy of the voting booth on Tuesday, they'll vote for much-needed changes to our failed marijuana laws."

Support for Prop. 19 is highest in the Bay Area and Northern California, and among voters under age 40. Roughly half of registered Democrats are in favor of the proposition, compared with just 25 percent of Republicans.

The most recent Field Poll also showed 48 percent of voters opposed to Prop. 23, the initiative that would suspend California's climate-change law. But 19 percent of voters are still undecided on the measure; 33 percent said they are in favor it.

Support for Prop. 25 has fallen dramatically since July, but is still far outpacing opposition. In July, 65 percent of people polled were in favor of the proposition, compared with 48 percent now. Thirty-one percent of voters are opposed to the proposition to allow the Legislature to pass budgets with a simple majority, and 21 percent are still undecided, according to the Field Poll.