Second- and third-generation Cuban-Americans born in the United States have continued to drift away from their parents’ and grandparents’ Republican Party. But, in a trend that went largely unnoticed by Democrats until lately, more recent Cuban immigrants who previously displayed little engagement in American politics have started to identify as Trump Republicans.

They are not enough to flip Miami-Dade, which Hillary Clinton won by a record margin of nearly 30 percentage points in 2016. But their potential impact to the race has led in part to an unusually pitched electoral battle in Florida’s most populous county this year, as President Trump’s campaign fights to narrow the Democrats’ lead and compensate for his expected losses elsewhere, including among older voters and suburban women.

If they can bring Joseph R. Biden Jr.’s advantage down to, say, 20 percentage points, the political math suggests that Florida, a must-win state for Mr. Trump, could remain in the president’s column, even if the Tampa and Orlando regions swing slightly toward Mr. Biden.