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Proposition 203, which would legalize the medical use of marijuana in Arizona, was on track to pass Friday after provisional ballots counted by Maricopa County pushed the "yes" vote into a commanding lead.

Unofficial returns showed Proposition 203 leading by 4,421 votes with about 10,000 ballots left to be counted, all of them in Maricopa County.

For the measure to fail, “no” votes would have to come in with a margin greater than 70 percent to 30 percent. That would be a big departure from how returns have come in from Maricopa County.

Andrew Myers, campaign manager for Proposition 203, said the tight race means supporters must show that a system dispensing medical marijuana will help patients but limit abuse.

"Our responsibility is to have a program that adheres to the highest of standards," Myers said.

Proposition 203 would allow a qualifying person with a doctor's recommendation to receive 2.5 ounces of marijuana every two weeks from licensed dispensaries. Qualifying conditions would include cancer, glaucoma, AIDS and chronic pain.

The ballot measure's fate had been in limbo since the Nov. 2 election, and the "no" vote led until late Friday afternoon.

The key to the turnaround: About 40,000 provisional ballots cast in Maricopa County and not counted until Thursday and Friday. Those came in for "yes" by a wide margin.

Carolyn Short, head of Keep AZ Drug Free, the measure's chief opposition, said she was glad the race was tight but found the outcome incredibly disappointing.

"I think many Arizonans are in for a big surprise when they find out what this is all about," Short said.

Supporters said the measure would offer relief to patients suffering from debilitating conditions.

Opponents argued that it would be the first step toward legalization and would lead to rampant abuse. They also argued that the measure, as written, offers special protection to marijuana users that insulates them from regulation.

The measure calls for the Arizona Department of Health Services to register and issue identification cards to patients and caregivers to use marijuana or grow up to 12 plants if they live far from a dispensary. The department will also regulate the dispensaries, of which there will be one for every 10 pharmacies.

Arizona voters approved medical marijuana use in 1996, but the measure never took effect because it would have required a doctor's prescription, which is illegal under the federal law.

Proposition 203 instead would require a doctor's recommendation, which have the same weight as a prescription but only on a state level.

U.S. Attorney General Eric Holder announced last year that the government wouldn't prosecute marijuana users who comply with state laws.

The Arizona Marijuana Policy Project, the main group supporting Proposition 203, raised $778,000. Three-quarters of that amount, or $579,000, came from the Marijuana Policy Project, a Washington D.C.-based lobby involved in many state marijuana laws.

Keep AZ Drug Free, 203′s chief opponent, raised more than $20,000 toward their cause. The Arizona Cardinals donated nearly half of that amount with a $10,000 contribution.

The proposition garnered little support from Arizona leaders. Five county sheriffs and 11 county attorneys state their opposition in the Secretary of State's Office publicity pamphlet.

U.S. Sens. John McCain and Jon Kyl, R-Ariz., joined at a news conference by Republican Reps. Trent Franks and John Shadegg, denounced the proposition.

Will Humble, director of the Arizona Department of Health Services, also stated his opposition but added his department would implement the system in a fair and efficient manner.

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