Maybe it’s something in the water — or this case, something not in the water.

Regardless of the cause, last week voters in Portland, Ore., decisively rejected a referendum proposal to add fluoride to their city’s water supply.

The 60-40 percent margin against fluoridation was achieved despite a 3-1 campaign- fundraising edge by the plan’s proponents — and despite its backing by the mayor, all four city commissioners, an assortment of health advocacy organizations and numerous minority leaders.

The outcome also flowed against the historical tide of mid-20th century decisions in a vast majority of U.S. metropolitan areas, including here in the Lowcountry, to add fluoride to public water supplies.

Back then, most opposition to fluoridation came from the political right.

Make that the hard right: The John Birch Society ludicrously decried putting fluoride in Americans’ water as a communist plot that advanced the insidious cause of “mass medicine.”

Yet Portland is one of the nation’s most politically “progressive” cities.

It’s also the only city among the nation’s 30 most populous to not put fluoride in its water: Tuesday’s vote was the fourth time Portland’s voters have flushed that notion.

And while that city’s voters have long leaned leftward in large numbers, their most recent fluoride fight, which generated combined campaign spending of more than $1 million, severely tested that sense of unity.

As the Portland Oregonian reported Wednesday, “stark, and heated, divisions” were exacerbated as the two sides ardently made their cases to the electorate:

“Both campaigns accused the other of stealing yard signs. A thinly veiled anti-fluoride push poll went out to voters. Opponents were described as insensitive to equity issues, while proponents were accused of wanting to willingly pollute the city’s famously pure water.”

But lest you fear that Portlandians got the fluoride decision right while we in the Charleston area got it wrong, consider these expert insights:

The American Dental Association hails fluoridation as “one of the safest and most beneficial, cost-effective public health measures for preventing, controlling, and in some cases reversing, tooth decay.”

The U.S. Centers for Disease Control ranks fluoridation No. 9 on its list of the top medical achievements of the 20th century. (Immunizations are No. 1.)

Still, on fluoridation for Portlandians, it’s their “City of Roses,” their water, their call.

After all, would you wants busybodies from afar ridiculing South Carolina voters’ judgments?