Proposition 19, a ballot initiative in California that would give local authorities the ability to legalize and tax marijuana for personal consumption, appears to have lost ground in the polls.

Calculating a trendline from all surveys on the initiative suggests that about 46 percent of Californians plan to vote in favor of Proposition 19 — a yes vote would legalize marijuana in the state — but 47 percent plan to vote no. This reflects a reversal from before, as the ballot measure had led in most surveys prior to this month.

The trend is fairly robust across different polling companies. Although SurveyUSA still has the yes vote favored, 48 to 44, their previous polls had shown the measure leading by a larger margin. The Public Policy Institute of California meanwhile, has the measure trailing by 5 points now after having led by 9 points before. And Ipsos, which had the “yes” side trailing by 2 points in its June survey, now has the initiative trailing by 10.

Proposition 19 may simply be getting swamped by the races for governor and Senate in California, both of which are quite competitive. Through Sept. 30, for instance, according to California’s campaign finance Web site, groups in favor of Proposition 19 had raised a total of about $860,000 in individual contributions, easily outdistancing the roughly $160,000 raised by groups opposed to it.

But these amounts are infinitesimal as compared with the tens of millions of dollars invested in the Senate and governor’s races, which is on the order of $200 million in the aggregate, much of it coming from the Republican gubernatorial candidate, Meg Whitman. Meanwhile, Ms. Whitman and the other three major-party candidates in California — Jerry Brown, Barbara Boxer and Carly Fiorina — are all opposed to Proposition 19. Interest in these races may be driving a traditional, medium-to-high midterm election turnout, which would tend to skew older. Since support for marijuana legalization is highly correlated with age, that could be harming the proposition.

Furthermore, there is a rule of thumb — although I haven’t seen much empirical study of it — that voters who tell pollsters they are undecided on a ballot initiative will tend to vote “no” on it on Election Day. This may because they are primarily interested in voting for other reasons — like the Senate and governor’s races — and will not have spent much time studying the measure, and therefore will default toward preserving the status quo.

Ballot measures, however, can be difficult to poll — particularly on culturally sensitive areas such as drug use. An analysis I conducted over the summer found some evidence that some polls could be underestimating the support that Latino and black voters are prepared to provide for the initiative, since marijuana usage can be especially stigmatized in their communities, and some of these votes could be reluctant to admit their support to pollsters.

It could also be the case that pollsters are under representing non-traditional voters, who may be inclined to vote solely because of Proposition 19. SurveyUSA, for instance, which is one of the few polls still to show Proposition 19 favored to pass, has found a group of “uniquely motivated” voters in California — people who might be weeded out (pardon the pun) by traditional likely voter screens, but who show a strong interest in this year’s elections. These voters favor Proposition 19 by a 7:4 ratio, according to SurveyUSA’s analysis, and could make the difference between its passage and its failure.

There are good reasons to think the polls could either be overestimating or underestimating Proposition 19’s support. 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.