The mayor of Miami-Dade County on Friday said she would veto 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 are present, and it is unclear whether there will be enough support. The next scheduled board meeting is on May 6.

Six of the 13 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.

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.”