SAN FRANCISCO (CBS SF) — Californians are just weeks away from a pivotal vote on the future of pot, but an exclusive new poll shows the cannabis race may be closer than anyone thought.

Voters approved medical marijuana use 20 years ago, now Prop 64 would legalize and tax the recreational use of marijuana. It’s one of the most controversial measures on California’s ballot this November.

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An exclusive KPIX 5/SurveyUSA poll shows 52 percent of voters say ‘yes’ to making pot legal, while 40 percent say ‘no’.

This is no slam dunk.

Usually with a controversial measure like this you like to have a seven to ten point edge because voters have a natural tendency to say no in the closing weeks of the campaign.

And our new poll shows, that edge may be going up in smoke.

With the support of big names like the California Medical Association, the NAACP and Lt. Governor Gavin Newsom, early polling showed six out of ten voters ready to vote to legalize recreational marijuana in California.

But the new KPIX 5/SurveyUSA poll of California voters shows support dropping to a much narrower 52 percent – with 40 percent saying ‘no’ – and eight percent undecided.

Emeryville police Chief Jennifer G. Tejada said, “This issue is not whether marijuana should be legalized, but how should it be legalized.

Support for full legalization appears strongest among baby boomers and with voters under 35 years old.

One voter told KPIX5, “People are doing it anyway, so just make it legal.”

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But from the get-go backers said the challenge would be with minorities.

One minority voter said, “It’s not good. I have people in my family whose brains are not well because of marijuana.”

Backers have been billing legalization as a legal and social justice issue as well.

Tenoch Flores, with the Yes on 64 campaign, said “Californians know that our current system with regards to marijuana is broken.”

One issue that both the state’s police chiefs and district attorneys are raising is driving under the influence.

Chief Tejada said, “If you look at Colorado and you look at Washington states, traffic collisions and fatalities have increased significantly.”

But Flores point out that, “The measure supports revenue so that law enforcement can administer the new law and develop a new standard for DUI in California.”

As for the new poll showing a close race?

Flores said, “Every other public poll that we’ve seen thus far has shown ‘yes on 64’ winning by a large margin.”

Other results from our poll include: a proposition to increase the tax on cigarettes to $2 per pack is enjoying strong support. 60 percent are backing Proposition 56, while 33 percent are against it.

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Our poll shows Proposition 63, which would ban large-capacity gun magazines and require background checks for those wanting to buy ammunition, is seeing 63 percent in favor and 29 percent opposed.