PORTLAND, Ore. - While soaking up the rays in what's been an unusually sunny season, Portlanders have broken away from their polite chatter about food, wine and outdoor adventure to fight about whether to fluoridate the water supply.

Supporters and opponents of public fluoridation have raised hundreds of thousands of dollars ahead of a Tuesday election that has drawn heavy attention from the city's mainstream and alternative newspapers.

Campaign signs have sprouted alongside roses on lawns across the city, as have reports of people stealing them.

Voters living in one of America's most liberal cities are generally in lockstep; rare is the political issue one feels hesitant to raise at happy hour. But fluoridation has emerged as one such topic.

Liberals concerned about the dental health of low-income children are pitted against liberals averse to putting anything unnecessary in the water. Added to the mix are libertarians who say fluoridation violates an individual's right to consent.

"This issue has brought together people who distrust government, folks that distrust industry and folks that are rabidly passionate about environmental causes," said Mike Plunkett, a dentist who backs fluoridation. "And those are very strange bedfellows."

Portland is the largest U.S. city yet to approve fluoridation to combat tooth decay. That changed ever-so-briefly in September, when the City Council unanimously voted to add the mineral to a water supply that serves about 900,000 people.

Opponents quickly gathered thousands of signatures to force Portland's fifth vote on the subject. Voters rejected fluoridation in 1956 and 1962 before approving it in 1978. The fluoride was never added because voters repealed their decision in 1980.

About three-fourths of the U.S. population drinks water treated with fluoride - three times the rate in Oregon. Most health organizations endorse fluoridation as safe, and the federal Centers for Disease Control listed it as one of the 10 greatest health achievements of the 20th century.

Yet, it was and remains a controversial subject throughout the United States, in places as different as Portland and Wichita, Kan., where voters soundly rejected fluoridation in November.

Opponents contend the dental benefits are relatively small and not worth marring water that tastes good. People with thyroid issues, kidney disease and multiple chemical sensitivities worry fluoridation will make their lives worse. Others are concerned it can cause cancer and child-development problems.

They say the pro-fluoridation camp considers the debate about its safety settled and ignores any study that raises questions.

"I do think that the science changes before policy, and eventually this is going to be an outdated, antique policy," said Kellie Barnes, a physical therapist and anti-fluoridation activist.

"It's not one IQ study or one bone-cancer study," she added. "But the emerging science, when taken in bulk, is showing there's a reasonable basis for concern. We're not saying: 'Oh, it's conclusive.'"

A recent survey showed 21 percent of Portland children between 6 and 9 have untreated dental decay - 6 percentage points higher than what was found in a similar 2010 survey of Seattle children who drink fluoridated water.