STUART, Fla. — A few steps away from the St. Lucie River, which has been choked lately with thick blue-green algae that made neighbors sick, Senator Bill Nelson of Florida sat solemnly with a group of elected officials, scientists and activists who had anxious questions about the toxic bloom. A day earlier, Mr. Nelson’s political challenger, Gov. Rick Scott, had begun airing an eye-catching television ad that blamed the senator and the federal government for failing to prevent the environmental crisis.

The meeting on Friday was Mr. Nelson’s second visit in a month. And this time, he declared, he was fighting back — not against the algae, but against the governor.

“I was playing nice-nice when I was here before, but I’m going to lay out the truth,” Mr. Nelson said. “Governor Scott, in the last eight years, has systematically dismembered and dismantled the environmental agencies of the state of Florida.”

Mr. Nelson, rarely a firebrand, then returned to his wonky element, delivering a long history lesson on water management and environmental policy. His audience nodded in agreement. But only a dozen people sat at the table listening to him. Mr. Scott’s ad, on the other hand, with its alarming footage of contaminated Lake Okeechobee juxtaposed with an unflattering image of Mr. Nelson, would reach thousands of potential voters.