TALLAHASSEE (Reuters) - Opponents of a proposal to let doctors in Florida prescribe marijuana are poised to launch a $1.6 million advertising campaign on Monday, with potential side effects for the state's heated race for governor in November.

Political consultants say a public battle over the proposed constitutional amendment to allow medical marijuana, included on this year's midterm election ballot, could boost turnout among young voters for Democrat Charlie Crist, who supports the measure. The extra support may be enough to help him defeat incumbent Republican Governor Rick Scott, who opposes it.

But opponents of Amendment 2 got a major boost when Sheldon Adelson, Las Vegas casino magnate and a major Republican fundraiser, put up $2.5 million of the $3.2 million raised by Drug Free Florida.

The ads will attack "loopholes," which law enforcement authorities say could lead to recreational use of the drug, said Sarah Bascom, whose Tallahassee consulting firm is handling the Drug Free Florida campaign.

"Our message is, this is not about sick people," she said. Even if more young voters turn out for the marijuana referendum, Bascom said "a vote down-ticket doesn't mean they're going to vote up-ticket" in the governor's race.

Some 23 U.S. states and the District of Columbia already have passed laws allowing medical marijuana in some form, beginning with California in 1996.

The public petition campaign that put the amendment on the Florida ballot promised relief for suffers of diseases like cancer, AIDS and ALS, also known as Lou Gehrig's disease. But the measure would also allow doctors to prescribe marijuana whenever they determine benefits outweigh potential risks.

The no campaign is being supported by wide-ranging "Don't Let Florida Go To Pot" coalition of medical and law-enforcement organizations arguing that the amendment does not provide any minimum age or residency requirements for obtaining marijuana.

Orlando trial lawyer John Morgan, a Crist backer who figures on spending $6 million of his own money on the medical-marijuana initiative, believes the referendum will draw up to 4 percent more voters to the polls Nov. 4, including college students who believe marijuana is no big deal, elderly voters who have friends who covertly puff some pot to relieve nausea from chemotherapy and libertarians who believe it is none of the government's business.

Morgan, who has been touring the state in a motorcoach, said the amendment might help Crist in what is expected to be a close race, but denied sponsoring the drive for that reason.

"We're registering people who never registered before, so I think turnout is going to be 2, 3, maybe even 4 points higher than we might have seen, but for this," he said during a visit to the University of Florida last week.

While supporting the amendment, Crist has distanced himself a little from the medical marijuana campaign.

"It might get more younger voters to turn out," Crist, a former Republican who was governor 2007-11, said in an interview. "I really don't know is the honest answer," he added. "It may well increase turnout because if you're educating people about this being on the ballot, who otherwise might not have known it, then you're ginning it up."

Supporters of the amendment say polling shows nearly 70 percent of likely voters back the measure with only 28 percent opposed.

But a recent Public Policy Polling survey showed just 61 percent support and a poll by the Tampa Bay Times/Bay News 9 and the Bob Graham Center at the University of Florida showed only 57 percent backing. Florida constitutional amendments require 60 percent support to pass.

Steve Schale, a member of the Crist campaign and a senior aide in President Barack Obama's two Florida campaigns, said amendment supporters might favor Crist more than Scott, but jobs, education and many other issues will be more decisive.

David Johnson of Strategic Vision in Atlanta, a Republican campaign planner, said it was hard to see young voters turning out in large numbers over one issue.

However, he cautioned, "Remember, even an uptick of 1 or 2 percent in a close race could make the difference."

(Editing by David Adams and David Gregorio)