SEATTLE » Most efforts to legalize marijuana possession have generally run aground in the face of unified opposition. Mothers Against Drunk Driving led the charge in helping to defeat a ballot measure in California in 2010. Law enforcement groups, not too surprisingly, have also been largely opposed in the past.

But in Washington state, as a measure that would legalize possession of small amounts of marijuana heads toward a vote next month, the opposition forces have been divided, raising hopes by marijuana advocates of a breakthrough. A poll conducted last month by Elway Research showed that 50 percent of voters either definitely or probably were in favor of legalizing the possession of an ounce of marijuana or less.

Some former law enforcement officials have appeared in television ads in favor of the legalization. Safety concerns about drugged driving have been muted by a provision of the measure, called Initiative 502, that would create a standard to measure impairment. A promised flood of tax money to drug and alcohol treatment programs from legal marijuana sales has also kept some anti-drug groups on the sidelines.

But if opponents are in disarray or disagreement, supporters of legalization are as well. And that is making the outcome hard to predict, both sides say.

In fact, some of the most vehement opposition to the initiative is coming from what might seem the least likely corner of all: medical marijuana users. Organized through a group called No on I-502, they say the plan, especially the new legal standard of impairment while driving, creates a new legal risk for regular users because THC, marijuana’s primary psychoactive ingredient, can stay in the bloodstream for days after consumption, and thus be measurable by a blood test whether a person is impaired or not.

At a recent debate at the University of Washington here in Seattle, a medical marijuana provider and the leader of the No on I-501, group, Steve Sarich, brought out a marijuana plant as a prop, but spoke to the mostly student crowd as if he were trying to scare them straight.

"They can take you to the hospital, they can take your blood," Sarich said about the driving provisions. "And if they find any trace of THC in your system, there goes your Pell grant, there goes your college."

On the opposite side of the debate, speaking on behalf of the measure, sat the Seattle city attorney, Pete Holmes, whose office prosecutes misdemeanor cases. Asked by an audience member why the initiative’s authors had seen the need for a crackdown on drugged driving after years of marijuana use in society, Holmes said the language was at least, in part, a political calculation.

Skeptics in California, he said, united around the lack of a provision about driving under the influence, and a ballot measure in Colorado this year appears to be facing the same headwinds.

"An initiative that doesn’t pass is a worthless initiative," Holmes said.

Part of the complication in Washington’s debate is that although at least 23 states have laws that address driving under the influence of drugs, they were all passed by legislatures rather than voters, according to the National Organization for Reform of Marijuana Laws, or NORML, a nonprofit advocacy group. That has introduced in Washington, supporters and opponents say, a complexity into the discussion that goes beyond the general question of whether marijuana should be regulated rather than being banned entirely.

But the infighting itself is proving to be its own political circus.

Last month, for example, Sarich and his allies lodged a complaint with the state attorney general’s office of illegal campaign activities by NORML and "scurrilous smear campaigns against those that oppose the NORML party line."

The executive director of NORML, Allen St. Pierre, called the complaint "goofy."

"They seem so half-witted that maybe they have filed the lawsuit against the wrong entity," St. Pierre said. "We haven’t sent any money, we haven’t raised any money. All we’ve done is the board voted unanimously to endorse an initiative."

As for the concerns about drugged-driving provisions, St. Pierre agreed that the language was not ideal. But there will be a blizzard of details to address in the Legislature once the voters have spoken, he said.

"Everybody anticipates if this passes, within a year or so afterwards it will be fixed," he said.

Drug treatment centers are also caught in uncertainty. Under the proposal’s language, they would get additional money from marijuana taxes. But at the Recovery Centers of King County, a spokesman said he thought it would probably be a wash in the end. Greater availability of any drug — the end of alcohol prohibition in the 1930s being a case in point, he said — leads to more abuse of it.

"Legalization will increase the rate of addiction to that substance," said Rome Doherty, an outpatient coordinator. He said the organization as a whole had taken no position on Initiative 502.

"But as a person, as long as there’s a limit and a roadside test for marijuana, I’m OK with it," he said.