A proposal to fluoridate Bellingham’s water supply was narrowly leading in early returns Tuesday night.

The fluoridation measure had about 51 percent of the vote by 9:25 p.m. About 9,100 votes had been tallied.

“I’m certainly hoping that will open up a little bit, but we’ll take 51 percent,” said Curt Smith, co-chair of Bellingham Families for Fluoride.

“I think everyone assumed it was going to be close,” he said.

The trend was disappointing, but the votes were far from counted, said Danelle Weaver, treasurer of Healthy Goals for Bellingham, which opposed the measure.

“At this point, I’m really not disappointed,” she said. “We have come a lot further than anybody expected.”

Fluoridation, Proposition 1, was the most controversial ballot measure in the city of Bellingham, and the most expensive election issue in the county’s history. Backers raised more than $250,000 to pass it, while opposition groups countered with more than $12,000.

Bellingham’s community debate over fluoridation has drawn national attention. Supporters cited scientific studies showing fluoridation is safe and stressed it was endorsed by many key government bodies and health organizations. They also pointed to a recent study showing 22 percent of Bellingham third-graders had cavities in seven or more teeth, compared with 8 percent in partially fluoridated King County.

But the opposition cited studies raising questions about whether fluoride is linked to several health issues, including a rare form of bone cancer.

Opponents also said putting fluoride in the water takes away people’s rights to decide whether to take it.

It would cost $600,000 to begin fluoridating the city’s water supply, according to an engineering study commissioned by the Washington Dental Service Foundation. The foundation has promised to cover the startup costs. Ongoing costs were estimated at about $30,000 a year.