Oral health and politics collided Tuesday as Portlanders _ proudly nonconformist and environmentally minded _ decided whether their city remains the largest in the U.S. without public fluoridation.

Voters had weeks to make their choice in the mail-ballot election. But by Tuesday it was too late to rely on the postman, so drop boxes have been placed across the city to accommodate those who waited until the final day to make a decision, as well as people who didn't want to pay for postage.

Supporters and opponents of fluoridation have raised hundreds of thousands of dollars and traded accusations of sign-stealing and shoddy science in an election that has been the city's most contentious of the 21st century.

If voters say no, Portland will remain the largest U.S. city without fluoride in the water or plans to add it to combat tooth decay. Voters in Portland twice rejected fluoridation before approving it in 1978. That plan was overturned two years later, before any fluoride was ever added to the water.

A sampling of voters dropping off ballots in rainy Pioneer Courthouse Square found people opposed to fluoridation.

"People don't like change. When in doubt, say no," said Tracy Rauscher, a native Portlander who, like a native Portlander, did not use an umbrella.

Portland's drinking water already contains naturally occurring fluoride, though not at levels considered to be effective at fighting cavities. Backers of fluoridation say adding more of it to the water is a safe, effective and affordable way to improve the health of low-income children whose parents don't stress proper nutrition and dental hygiene.

Opponents describe fluoride as a chemical that will ruin the city's pristine water supply, and they argue that adding it would violate an individual's right to consent to medication.

Although most Americans drink water treated with fluoride, it has long been a contentious topic. In the 1950s, fluoridation was feared as a Communist plot. Today, people worry that its effect on the body has not been sufficiently examined.

"I don't want chemicals in my water," Sarah Lazzaro said after voting Tuesday. "I know that there are really no known health risks with it, but there's a lot of things we find out later in life really do have health risks."

The issue re-appeared on Portland's radar late last summer, when health organizations that had quietly lobbied the City Council for a year persuaded the panel to unanimously approve fluoridation by March 2014.

Days before the vote, 227 people _ most of them opponents _ signed up to testify at a public hearing that lasted 6 1/2 hours. When their objections weren't heeded, they quickly gathered tens of thousands of signatures to force Tuesday's vote.