If Mr. Biden is able to make inroads across the state’s Republican-rich retirement communities with voters who regret supporting Mr. Trump or voted third-party in 2016, it would greatly complicate the G.O.P.’s arithmetic. And should the president perform better with Hispanics than he did four years ago, and cut deeply into Mr. Biden’s advantage in urban areas like Miami, it would all but block any Democratic path to victory in Florida.

It’s a departure from an earlier era, when the key to claiming this polyglot political jigsaw puzzle was wooing voters along the I-4 corridor across the middle of the state. Candidates from both parties beat a path to that fabled stretch of highway because the electorate around Tampa and Orlando was up for grabs.

Now, though, with surveys indicating that over 90 percent of voters know who they are supporting, the race could be decided by who does a better job turning out those who have already decided.

No voters appear more decisive than seniors, who polls show are more amenable to Mr. Biden than they were Ms. Clinton, and Hispanics, who the same surveys indicate are more supportive of Mr. Trump than they were in 2016.