The debate flared on Thursday at a public hearing before the five-member City Council, whose vote on the issue is scheduled for this week. Some of the arguments have focused on health concerns, with opponents raising questions and citing studies about fluoride’s effects on brain development and I.Q.

Others said that times had changed since in the 1950s, when fluoridation first took off in the United States. People now get plenty of fluoride, critics say, in products from toothpaste to food that has been processed, grown or cleaned elsewhere. About three-fourths of the country’s population lives in areas where fluoride is added to the water.

“If there’s a problem with kids’ dental health, why don’t we put the money toward better nutrition?” said Angel Lambart, who testified on Thursday with her young daughter on her lap.

But befitting a city where socially conscious, environmentally fervent goals have become a kind of local brand, the more ferocious questions for both sides centered on personal choice and societal burden. Once consigned to the ranks of the gritty and damp, along with Seattle and other Pacific Northwest shipping towns, Portland has been reinvented by waves of highly educated newcomers working at places like Intel, a huge regional employer.

But beyond the bike-friendly, walk-friendly downtown of rose gardens and coffee bars, a kind of parallel Portland of perpetual poverty, concentrated in minority groups and often invisible to visitors, has festered.

Dental decay rates, numerous state and federal studies say, are linked to income, education and access to health insurance, but also to lifestyle, diet and parental choices in insisting on a toothbrush. Such conclusions give fuel to both sides, with supporters of fluoride seeing a social problem to be solved by government, while opponents focus on unhealthy habits and diets that they say will not be affected by chemicals.

Mayor Sam Adams, who backs the fluoridation plan and has one of five votes on the Council, comes firmly down on the side that says Portland must address dental care for everyone if its progressive goals and self-image are valid.

“It’s about health equity, it’s about social justice,” he said in an interview. “Fluoride is means to an end,” he added. “I hope that folks, whether they agree with me or not, understand that my intentions are to help those Portlanders that have no voice in this process.”

Some opponents of fluoridation have lobbed that kind of reasoning right back into the laps of the commissioners. “I don’t appreciate you trying to alleviate your white guilt by putting toxins in our water,” Frances Quaempts-Miller, who described herself as mixed black and Muscogee Indian, said in testifying at the public hearing before the all-white Council of four men and one woman.

Who is empowered to make the choice for Portland has become part of the debate as well. Fluoridation efforts have failed at the ballot box here, most recently in 1980. And two other members of the City Council, in addition to Mr. Adams, who is not seeking re-election in November, said even before public testimony was taken that they planned to vote for the plan — making a majority — when it goes before them. Some opponents call that an undemocratic end-run around the popular will.

But one of the commissioners, Nick Fish, in a defense of his pro-fluoride position at Thursday’s hearing, said that what some call closed-door deal-making, he calls leadership. “I think we were elected to make the tough calls,” said Mr. Fish, the public works commissioner. He said that gridlock in Congress was an example to be avoided. “In this community we have a chance to do something different,” he said.

Some critics say Portland’s sense of civic duty is misplaced when it comes to bad teeth, which they say is a much greater problem in what many residents call the “other Oregon,” beyond the Portland metro area. Portland has bounced back from recession, while much of the rest of the state, where residents also mostly drink unfluoridated water, continues to struggle.

But even before the recession, there was a dental divide. A study in 2007 by the Oregon Department of Human Services said that 1 out of 17 first, second and third graders outside the Portland metro area needed urgent dental care because of pain or infection, but in the metro area itself, only 1 in 100 students did. Adding fluoride to Portland’s water would affect about 900,000 residents in a state of 3.9 million people.

“Portland has better teeth,” said Kim Kaminski, director of Clean Water Portland, a group that is fighting fluoridation. She called fluoridation for Portland a Band-Aid on a larger problem.

Fluoride remains a debating point around the nation, with some communities questioning the cost or re-examining the science. In Florida, the Pinellas County Board of County Commissioners voted last October to halt fluoridation, but the water district serving San Jose, Calif., voted in December to start.

Portland residents interviewed on a downtown public square on a morning last week were divided on whether voters or their elected representatives should have the final say.

“There are issues that are more complex than the general population sometimes can really assess appropriately,” said Carol Lindstrom, the marketing director for an energy efficiency company. Ms. Lindstrom said she grew up in a rural community in Washington State that did not have fluoride, “and I think I suffered for it,” she added.

Celia Wagner, a Web site developer, said the local fluoride debate, full of hyperbole and passion, had brought out the “rugged individuality and quirkiness” of Portland’s character, for better and worse. “It’s Portland doing its Portland thing, which I love; I’ve been here 35 years,” she said. “It’s charming, but occasionally not charming.”

Still, she said that it should be the voters who decide, and that she would vote for fluoride if she could. She grew up with it in the suburbs of Seattle. “People tell me I have nice teeth,” she said. “I don’t see the problem.”