The same studies indicate that felons were highly likely to register as Democrats (African-Americans, a strong voting bloc for Democrats, are a disproportionate share of the disenfranchised.) That would probably be enough to cover Mr. Trump’s 113,000-vote margin of victory from 2016, and perhaps quite a bit more.

But it probably wouldn’t mean a significant and durable edge for Democrats. Just ask Hillary Clinton, who inherited a 90,000-vote advantage from Barack Obama in the 2012 Florida election — President Obama defeated Mitt Romney, 50 percent to 49.1 percent — and seemed to benefit from four years of demographic shifts, including an influx of Puerto Rican voters, and still lost the state by more than 100,000 votes.

One reason is the overlooked influx of fairly conservative white voters. Florida has long been a retirement destination, and its above-average number of high-turnout older white voters have both trended toward the Republicans and helped counter other demographic shifts.

The Villages, Fla., was the fastest-growing metropolitan area in the United States in 2016; it voted for Mr. Trump by 39 points. Over all, 10 of the 25 fastest-growing metropolitan areas in the United States were in Florida in 2016 , and all except Orlando-Kissimmee — where there has been considerable Puerto Rican immigration — supported Mr. Trump in 2016.

In 2017, newly registered Floridians registered as Republicans by a margin of 29 percent to 27 percent over Democrats, according to data from L2, a nonpartisan election data vendor. That’s despite the uptick in Hispanic registration after Hurricane Maria hit Puerto Rico on Sept. 20 (Hispanic voters represented 22 percent of registrants in the two months before Maria, and 29 percent in the two months after). White voters over age 50 registered as Republicans by a margin of 45 percent to 21 percent.

Another reason: Seemingly big demographic changes can easily be swamped by even slight shifts among the rest of the electorate. If President Trump were to gain another percentage point among white working-class voters, for instance, that could be enough to overcome the combined effect of both new Puerto Rican residents and felon re-enfranchisement. Persuading a voter to switch sides is twice as helpful as turning out a new one: Persuasion both takes away a voter from your opponent and gives you one. Adding a new voter does only the latter.

Over the last two decades, Florida has been the closest state in the country in presidential elections. That may change one day. But if it does, it probably won’t be because of a single, modest demographic shift, like felon re-enfranchisement or an influx of Puerto Rican voters. Florida is the nation’s third-most populous state. It takes a lot to move the needle.