LONGBOAT KEY, Fla. — Four years after President Trump won Florida by just over a percentage point, polls show that the state is, true to form, sitting on a knife’s edge — and looming again as a potential tipping point.
On Tuesday, Joseph R. Biden Jr. will make his first trip to the state since claiming the nomination last spring. Mr. Trump, guarding a state that no Republican president has lost since Calvin Coolidge, has made a number of visits to the state, including last week.
In an era of polarization, where swing voters are scarce, elections in Florida are won by driving up turnout among the faithful and running up margins in favorable terrain while losing more closely in hostile precincts. In a state so evenly divided that races are often decided by a few thousand votes — or, more memorably, a few hundred — mobilizing the converted outweighs preaching to the undecided few.
“The secret to Florida now is that it’s a margins game,” said Gwen Graham, a former congresswoman who worked for decades in the campaigns of her father, the governor-turned-senator Bob Graham.
If Mr. Biden is able to make inroads across the state’s Republican-rich retirement communities, 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.