A constitutional amendment to legalize medical marijuana in Florida is in serious jeopardy.

For more than a year, the amendment seemed to enjoy broad support, cutting across political, racial and age lines.

But with opposition forces financing TV ads and sheriffs showing up at forums, support for the amendment has slipped dramatically, according to a new Tampa Bay Times/Bay News 9/UF Bob Graham Center poll.

Only 48 percent of likely voters said they would vote for Amendment 2. Forty-four percent oppose it and 7 percent said they had not made up their minds.

The requirement that it pass by a 60 percent vote now represents an imposing hurdle.

"My guess today is this is not going to pass,'' said David Colburn, director of the Bob Graham Center for Public Service. "It may not mean that Floridians don't support the use of medical marijuana,'' he said, but apparently many voters dislike the amendment's wording and embedding it into the state Constitution.

Amendment 2 would allow people with disabling conditions to possess pot if a doctor certifies that they need it.

The telephone survey of 781 registered Florida voters — all likely to vote in the November election — was conducted Oct. 7-12 for the Tampa Bay Times, Bay News 9 and News 13 of Orlando by the University of Florida's Bob Graham Center for Public Service and Bureau of Economic and Business Research. The poll, which included respondents using landlines and cellphones, has a margin of error of plus or minus 3.2 percentage points. Results were weighted by age, party registration and media market, thus allowing the results to mirror the distribution in the Florida Voter File.

Just six weeks ago, the Times partner poll found a much different result, with 57 percent saying they would vote in favor and only 24 percent saying no. That poll gave the option of "Haven't thought much about this," which yielded a large group of undecided voters — about 17 percent.

With 44 percent of likely voters now polling against, opponents may block the amendment even if they do not attract a single remaining undecided voter.

Medical marijuana opposition includes St. Petersburg's Drug Free America Foundation, the Florida Sheriff's Association and the Florida Medical Association. In ads, town hall meetings and debates they contend that the amendment is loosely worded, and will amount to de facto legalization of recreational pot.

"My sense is that people are organized against it far more than those favoring it,'' said Christopher McCarty, director of the UF Survey Research Center and director of the University of Florida's Bureau of Economic and Business Research.

Other than Orlando lawyer John Morgan, who is barnstorming the state for medical marijuana, supporters are "a diffuse set of people,'' McCarty said, "and they don't come across with an organized message like the sheriffs and the medical association.''

Louise Shinkman, 75, a Fort Myers real estate agent and political independent, is voting no.

"I disagree with the part about caregivers who don't need to be licensed or checked or anything,'' said Shinkman, repeating language from a recent television ad opposing Amendment 2. It claimed that even drug dealers could be caregivers and caregivers would need no training.

No state requires special training for medical marijuana "caregivers" because they are not medical personnel. Some patients are too disabled to buy or grow marijuana for themselves so all states allow designated "caregivers" to act on their behalf. The amendment allows the Legislature to draft regulations about who can be a caregiver.

Voters interviewed about their support for the amendment often recounted personal experiences with sick people.

"My mom had breast cancer,'' said Carrie McRay, 39, a Republican sales associate from Homosassa. "The only way she could even eat during chemotherapy was to have something to smoke.''

Though early polls showed nonpartisan support for medical marijuana, the Times/Bay News 9/News 13 poll shows sharp divisions along party lines. Sixty percent of registered Democrats said they would vote yes, 61 percent of Republicans said they would vote no, and 55 percent of independents favored it.

Differences also showed up among age groups, with 74 percent of the 25-to-34 bracket in favor and 56 percent of voters 60 or older opposing.

Times staff writers Weston Phippen and Katie Mettler contributed to this report. Contact Stephen Nohlgren at nohlgren@tampabay.com.