The veto by Mayor Daniella Levine Cava, a Democrat, pushed back on a growing campaign against the mineral, which has been used for decades to prevent cavities.

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.