An anti-fluoride group said it’s been so successful collecting signatures to block Portland’s controversial fluoride plan that it will turn them in Thursday — a day ahead of schedule and with thousands of signatures to spare.

If they succeeded in collecting 19,858 valid signatures from city voters, Portland’s already-approved fluoride plan would come to a screeching halt. It wouldn’t move forward unless voters approve, with the most likely election date not until May 2014.

Clean Water Portland collected more than 35,000 signatures by Wednesday afternoon, leader Kim Kaminski said, and the group plans to submit them Thursday afternoon, ahead of Friday’s 5 p.m. deadline.

“I am confident that we will get this on the ballot,” she said. “If I weren’t confident, we wouldn’t be turning them in a day early.”

Portland’s long and contentious history with fluoride dates back nearly 60 years, with residents supporting the cavity-fighting chemical once but voting against it three times. Before the City Council unanimously approved its plan Sept. 12, Portland was the largest city in the country that hadn’t taken steps to add fluoride to its water supply, which also serves residents in Gresham, Tigard and Tualatin.

City Commissioner Randy Leonard, who pushed the Water Bureau to add fluoride by March 2014, conceded Wednesday that all planning work will stop if the signature-gathering is successful. But he said he isn’t disappointed.

“That’s why the process exists,” he said, adding that he expects a “full and meaningful debate” in the months ahead.

Ironically, that’s been a major complaint about Portland’s process. Mayor Sam Adams and Commissioner Nick Fish joined Leonard in announcing their support of fluoridation weeks before public meetings or casting votes. While some residents fervently oppose fluoride, others were upset by city leaders’ failure to let voters weigh in.

Since the council vote, Clean Water Portland’s referendum committee has collected more than $41,000 and paid dozens of people to collect signatures. Referendums challenge a City Council ordinance but give opponents just 30 days to collect signatures from at least 6 percent of registered voters.

“Given the short amount of time that we have, and given the lack of notice by City Council and the way they railroaded it through, we had really no choice but to hire paid signature gatherers,” Kaminski said.

It’s been 11 years since a group successfully challenged a city decision. In 2001, the City Council voted to establish street fees, but big businesses — which would have faced big bills — helped bankroll a signature-gathering effort. The City Council repealed its decision instead of sending it to voters.

“It’s not a typical process for the city, so we don’t have a lot of history to go of off,” said Deborah Scroggin, the city’s elections officer.

Once Clean Water Portland submits its paperwork, city and county officials will have 30 days to certify whether enough of the signatures are valid. From there, the council has another 30 days to consider whether to repeal its ordinance, although that scenario is unlikely.

That means the fluoride issue would head to voters in the May 2014 primary election unless the council pushes for an earlier decision. According to city code, the council can set an earlier date if the “public interest in a prompt resolution of the question outweighs the costs associated with a special election.” Among possible options are March, May, September or November 2013.

Fluoride supporters said they want to wait until all the signatures are verified before talking about next steps. But the Everyone Deserves Healthy Teeth Coalition has proved politically savvy through lobbying efforts at City Hall and in a television ad campaign.

“We absolutely will continue to promote fluoridation as the right step for our city to improve the dental health of Portland children and families,” said Raquel Luz Bournhonesque, who organized the coalition.

Kaminski said voters deserve a chance to decide for themselves.

“It puts everything on hold,” she said, “so that people have time and step back and take a deep breath and figure out what’s going on.”