Medical marijuana enjoys broad backing in Florida, with polls in the past year indicating that 65 to 70 percent of voters support the idea.

But passing a constitutional amendment to legalize medical pot may not be the cakewalk that such numbers suggest.

Florida requires a 60 percent majority to amend the state Constitution. Older voters — who usually dominate turnout — favor medical marijuana, but not as strongly as younger voters do.

Most important, experts say, amendments historically lose support as campaigns heat up and opposition weighs in.

"What I tell clients is that you want to poll at least about 10 percent higher (than the 60 percent threshold) out of the gate,'' said Orlando consultant John Sowinski, one of Florida's ballot initiative gurus. "It's like you are cut and bleeding on day one, and it depends on whether or not you are a big bleeder.''

Tallahassee consultant David Johnson, who generally works for Republicans, found 65 to 70 percent support for the amendment in his own personal polling but thinks that will dwindle.

"It's going to be close,'' he said last week.

The amendment's ballot language can be interpreted as offering easy access to pot, Johnson said. "What people do not want to see in Florida right now is any steps toward'' full legalization.

But "if people are thinking it's for someone who is very ill,'' he said, "then it will pass.''

Republican consultant Rick Wilson, also of Tallahassee, thinks the cost of mounting an effective statewide campaign will help the amendment pass, but by a small margin.

The Florida Sheriff's Association, the Florida Medical Association and other opponents will carry moral weight, Wilson said, but "I don't think they are going to be spending much money.

"The rule in Florida is that you are going to have to spend a million dollars a week to make the case on a ballot initiative.''

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Pro-amendment forces enjoy the deep pockets of Orlando lawyer John Morgan, who spent nearly $4 million to get medical marijuana on the ballot. Spending for the campaign will depend on opponents, Morgan said. "We will see how much offense there is to see how much defense we have to put up.''

Morgan is planning TV ads in September and October "with real people who have real-life experiences with diseases.'' Campaign director Ben Pollara expects a budget of at least $10 million.

If the campaign gets expensive, so be it, Morgan said. Among other things, he said, he has discovered that publicity over medical pot brings in clients.

"People like a fighter,'' he said. "I've been above the fold twice in every major newspaper in Florida. I put that into the equation when I decide what to do.''

Morgan's group, United for Care, has received contributions from hundreds of small donors, but he also expects support from "growers and people who want to open dispensaries,'' he said. "This is big business.''

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Sowinski said his firm, Consensus Communications, could end up working against the amendment. He suggested that Colorado, which has legalized recreational marijuana, might prove to be the "X factor" in the Florida campaign.

Voters in other states have rejected the argument that medical marijuana is a stepping stone to full legalization, Sowinski said.

"That sounded far-fetched a year ago," he said. "Now, not so far-fetched.''

Doug Kaplan, founder of Gravis Marketing in Winter Springs, thinks Colorado might help the Florida initiative if no horror stories emerge over the next eight months — "if people see legalization in Colorado and the world doesn't end.''

He found only 57 percent support for the measure using the amendment's ballot language.

Kaplan still thinks the amendment will squeak by if national attitudes toward pot keep shifting as the campaign progresses.

"I never thought I would see acceptance of medical marijuana,'' Kaplan said. "It's amazing how society is changing.''

Wilson noted that many conservative politicians are distancing themselves from the fight. He advises his Republican clients to avoid coming across as if they "support criminalizing seriously ill and dying people,''

Gov. Rick Scott, for example, "is not talking at all about this at all, and that is very telling,'' Wilson said.

Morgan thinks the medical pot issue can help Charlie Crist, one of his employees, win the governorship, Wilson said. And Scott needs to ignore that bait.

"If this election turns into a discussion about (pot) … that's a bad dialogue,'' Wilson said. "The economy is a good dialogue."

So far, Scott has taken a tepid position. He personally opposes medical marijuana, he says in statements, but then he adds: "No matter my personal beliefs, however, a ballot initiative would be up to the voters to decide."

Stephen Nohlgren can be reached at nohlgren@tampabay.com.