If this is Friday, California's ballot measure to legalize marijuana use for adults must be ... losing?

That, indeed, is the result of the latest poll on Proposition 19. A Los Angeles Times/USC poll released early Friday shows likely voters oppose the ballot initiative by 51 percent to 39 percent, with 8 percent undecided or declining to state their intent.

However, there are a lot of "howevers." Foremost is that poll results on this particular initiative have seesawed wildly – reflecting either fast-changing views of Californians about legalizing marijuana or, perhaps as likely, the difficulty of getting accurate survey results on this issue.

Four polls conducted in September, for instance, had Prop. 19 winning. This month, four polls have it losing and one poll has it winning. In all of those polls but one, the "undecideds" were at least 7 percent – and in most cases that undecided number was bigger than the win-loss point margin in the poll.

That, coupled with suspicion that pro-legalization respondents may not be telling pollsters their true feelings about the measure, makes it hard to tell what's really going on with California voters.

"There are very good reasons to think the polls could either be overestimating or underestimating Proposition 19’s support,” writes Nate Silver, a blogger for the election forecasting site FiveThirtyEight.com. “In spite of the recent trends against Proposition 19, therefore, I would be inclined to take the recent polling at face value, which suggests that the measure has about even odds of passing.”

The Los Angeles Times/USC poll, conducted by the Democratic firm Greenberg Quinlan Rosner and the Republican firm American Viewpoint, surveyed 441 likely voters by phone, both cellphone and land lines, from Oct. 13 to 20. It had a 4.6 percent margin of error.

Here are some explanations both sides offer for the apparent shift in voter sentiment against Prop. 19:

• Two days into the polling period, US Attorney General Eric Holder came to California to announce that the Obama administration would “vigorously enforce” federal drug laws against people who sell, distribute, or grow marijuana for recreational use – thereby making clear that pot users would still face criminal sanctions no matter what happens with Prop. 19.

• Gov. Arnold Schwarzenegger recently signed legislation making possession of an ounce of marijuana an “infraction” (as opposed to misdemeanor), and voters may feel that a punishment that is the practical equivalent of a traffic ticket is appropriate – and all that needs to be done.

• Races for governor and Senate in California may be affecting voters' views on Prop. 19. Four major-party candidates – Jerry Brown, Meg Whitman, Barbara Boxer, and Carly Fiorina – are all opposed. These prominent races may drive turnout up, bringing out older voters who would be more likely to offset younger voters' enthusiasm for Prop. 19.

The No on 19 camp says now that voters have had time to scrutinize the initiative, more are rejecting it – the latest polls are simply reflecting that.

“As expected, California voters are taking a closer look at Prop. 19 and are just saying no,” said Roger Salazar, spokesman for No on Prop. 19.

He cites a Public Policy Institute of California (PPIC) poll conducted Oct. 10-17 that showed 49 percent of likely voters were against it versus 44 percent in favor – an eight-point drop in support from the PPIC's poll in September. “The PPIC survey shows Prop. 19 has not been able to spark up support among voters, especially Latinos or independents,” says Mr. Salazar.

“While the measure claims to regulate, control, and tax marijuana, voters don't need eye drops to clearly see it does none of those things,” he adds. “The chronic flaws in Prop. 19 endanger public safety, make a mockery of workplace safety rules, and won’t generate the revenue claimed by proponents.”

Still, Prop. 19 is uncharted territory. No state has ever held a vote to approve or reject legalizing marijuana, so it can be difficult for pollsters to pin down who is a "likely voter" in this election, notes Steve Fox, director of government relations for the Marijuana Policy Project, a national organization that works to increase public support for nonpunitive marijuana policies.

“This is an issue that can motivate the young, irregular voter like no other,” says Mr. Fox. His organization is seeing “incredible enthusiasm on [college] campuses” nationwide, with many volunteering time to make calls to voters in California on the proposition's behalf, he says. The vote is considered crucial to the future of the pro-marijuana movement nationwide. If passed, Prop. 19 would allow adults at least 21 years old to possess up to 1 ounce of marijuana and grow 25-square-foot pot gardens for personal use, and it would authorize county and city governments to regulate and tax commercial cultivation and sales.

Moreover, Fox says, certain polls may not be reflecting some voters' true intent to vote for Prop. 19. One thesis is that polls that use live operators to ask the questions register less approval for Prop. 19, because respondents are reluctant to be honest in front of an actual person. Those that use automated calls may not encounter that same resistance, the thinking goes. One poll this month, USA Survey, uses robocalls and had the yes vote favored 48 to 44 percent, with 8 percent undecided.