The mayor of Miami-Dade County on Friday said she vetoed legislation that would remove fluoride from the drinking water in Florida’s most populous county, pushing back on a growing campaign against the mineral used to prevent cavities.

The veto by Mayor Daniella Levine Cava, a Democrat, comes at a moment when critics of adding fluoride to the water supply have a newly powerful ally: Robert F. Kennedy Jr., President Trump’s health secretary, who this week called for states to ban fluoride in drinking water. Utah recently became the first state to outlaw adding fluoride to public water, and several other states, including Florida, are considering similar action.

“The science is very clear,” Ms. Levine Cava said at a news conference on Friday. She added, “ending fluoridation could have real and lasting harm, especially for children and families who cannot afford regular dental care.”

The Miami-Dade Board of County Commissioners, a nonpartisan body, approved the measure to ban fluoride on April 1 in an 8 to 2 vote, with three commissioners absent. Nine votes are needed to override the mayor’s veto if all 13 commissioners are present, and it is unclear whether there will be enough support. The next scheduled board meeting is on May 6.

Six commissioners are Republican, including Kevin Marino Cabrera, an ally of Mr. Trump’s who will soon depart to become the country’s ambassador to Panama. Ms. Levine Cava is now the state’s highest-ranking elected Democrat, with Republicans having swept every other countywide elected office in Miami-Dade — including sheriff and elections supervisor — last year.

At the news conference on Friday, Ms. Levine Cava, flanked by dentists and doctors in their white medical coats, cited research supporting her decision.

“I’m doing what I believe is the correct thing to do in the interest of the community’s health, and I stand with our dental and medical experts,” she said.

Commissioner Roberto J. Gonzalez, who sponsored the legislation, accused Ms. Levine Cava of “acting like a typical politician, relying on partisan pollsters and tired talking points, while putting people’s health at risk.”

In a statement on Friday, he called on his colleagues on the commission to override the veto.

The measure would require Miami-Dade to stop adding fluoride to the water supply within 30 days. Ms. Levine Cava said she and her office were closely following efforts in the Florida Legislature to pass a similar ban at the state level.

Many experts have warned that removing fluoride from drinking water would be detrimental to oral health, and particularly cavity prevention. The Centers for Disease Control and Prevention has called fluoridation one of the “10 greatest public health achievements of the 20th century.”

But wariness about fluoride has taken on new life in recent years, especially after the coronavirus pandemic eroded trust in public health interventions. Opponents say they want to protect bodily autonomy and have raised concerns about the possible cognitive effects in children.

They point to a recent review paper, where an analysis of 74 studies conducted by researchers with the National Institutes of Health found that there was a connection between lower I.Q. scores in children with high levels of childhood or prenatal fluoride exposure. (The levels studied were twice as high as those recommended by the C.D.C., and some research has found no link.)

Ms. Levine Cava’s veto puts her at odds with Mr. Kennedy and other fluoride skeptics in the Trump administration, as well as with Gov. Ron DeSantis, a Republican, and his appointed surgeon general, Dr. Joseph A. Ladapo, who has been traveling around the state to speak against fluoridated water.

“Do we want forced medication or do we want people to be able to choose medication?” Mr. DeSantis said recently. “When you are forcing it into the water supply, that’s not really giving people a choice.”

Ahead of the mayor’s announcement, Dr. Ladapo, who has also called for a halt to using Covid vaccines, publicly urged Miami-Dade residents to ask the mayor to support the fluoride ban. “I’ll never understand how anyone feels entitled to add drugs to the water that other people drink,” he said.

Fluoride was first introduced to a city’s water supply in 1945 and began to become common practice across the country in the following decades. Studies show a direct correlation between fluoridation and an improvement in oral health.

“The growing distrust of credible, time-tested, evidence-based science is disheartening,” Dr. Brett Kessler, the president of the American Dental Association, said in a statement this week. “When government officials, like Secretary Kennedy, stand behind the commentary of misinformation and distrust peer-reviewed research, it is injurious to public health.”

Fluoridating water has been debated for years, in part because experts say that excessive exposure to fluoride over a long period can cause health problems. Federally mandated levels have lowered over the years, including after a recent court order.

On Monday, Lee Zeldin, the head of the Environmental Protection Agency, announced a decision to “expeditiously review new scientific information on potential health risks of fluoride in drinking water.”

“Without prejudging any outcomes, when this evaluation is completed, we will have an updated foundational scientific evaluation,” Mr. Zeldin said in a statement. He credited Mr. Kennedy for having “long been at the forefront of this issue.”

At a Miami-Dade commission meeting in April, most people who spoke during the public comment portion were against fluoridation. Days after the meeting, Ms. Levine Cava held a round-table discussion with doctors, dentists and others in the community that focused on its benefits.

Nearly 20 other cities and county governments in Florida have voted to remove fluoride from their water supplies since the November election. Among those, Miami-Dade County, with about 2.7 million people, is by far the largest. A bill under debate in the Florida Legislature would bar local municipalities from adding fluoride to water.

Miami-Dade’s politics have shifted sharply to the right in recent years, reflecting Florida’s transformation from a battleground state into one that votes reliably Republican. In November, Mr. Trump became the first Republican presidential candidate to win the county since 1988.

Patricia Mazzei contributed reporting