Legalizing marijuana for adult use in Arizona has more supporters than opponents, but the idea has lost ground in Maricopa County in recent months, according to a new poll.

A committee funded by medical-marijuana dispensaries is hoping a majority of voters approve adult use next year, and recently filed the language of the proposed measure with the state.

The new poll from OH Predictive Insights of 600 likely 2020 voters showed 50% support such a measure, 40% oppose and 10% are undecided.

The specific question was: "Do you believe that marijuana should be legalized for adult use in the State of Arizona?"

The poll had a margin of error of 4 percentage points and was conducted Aug. 13-14.

A legalization effort failed in Arizona in 2016 by a narrow margin. Polls by the same firm ahead of the 2016 election never showed such strong public support.

The poll showed similar results to the same question when it was asked in February, and again in May, though this time around more people reported being undecided.

The February poll showed 52% in favor of marijuana legalization and 41% opposed. Just 7% were undecided at that time.

But a key difference between the polls is the decline in support from Maricopa County.

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In May, 52% of Maricopa County residents supported the idea, and that dropped to 44% in the most recent poll, tied with opposition in the state's largest county.

Meanwhile, support in Pima County and rural areas grew during the same time, according to Mike Noble, chief of research at OH Predictive Insights.

Noble also said that ballot measures generally need to poll at 60% or better to wind up winning on election day, and legalizing marijuana remains well below that threshold.

“It is a long way until November 2020, but backers of the marijuana initiative should be concerned just 50% of voters approve of legalization,” Noble said.

Reach reporter Ryan Randazzo at ryan.randazzo@arizonarepublic.com or 602-444-4331. Follow him on Twitter @UtilityReporter.