The Little Havana neighborhood in Miami. The growth of the Hispanic population in Florida poses a problem for Donald Trump. 

If demographics are destiny, Donald Trump’s political fate could very well be sealed in Florida.

The big demographic threat to the Republican Party isn’t a “blue” Texas or Arizona or Georgia, but the possibility that Florida will follow Nevada and New Mexico to the left. It’s extremely hard for a Republican to win the presidency without Florida’s 29 electoral votes.

The polls suggest that Hillary Clinton might capitalize on huge demographic shifts to an extent that Barack Obama never did. She might even lead by the same margin in Florida that she does nationally — about five percentage points — even though the state has been more Republican than the country in every presidential election since 1976.

To understand why Florida could shift so abruptly, consider this: The demographic changes in Florida over the last decade rival or even exceed those in states like Nevada or Virginia, but the Democrats haven’t gained nearly as much as they have in those states. When Al Gore and George W. Bush fought to an effective tie in the state 16 years ago, 78 percent of registered voters were non-Hispanic whites. When Barack Obama edged Mitt Romney in 2012 by less than a point, just 66.5 percent of registered voters were.

Other changes have also helped Democrats. Back in 2000, the state’s Hispanic voters, disproportionately Cuban-Americans, leaned Republican. The state’s Hispanic vote now clearly leans Democratic. The newest generation of Cuban voters is far more Democratic than earlier generations, who came of age in the Cold War.

At the same time, growing numbers of non-Cuban Hispanics, particularly those from Puerto Rico, have pushed the state’s Hispanic vote further to the left.

But despite all of these shifts, Florida remained a state that tilted just slightly to the right of the country. That’s because demographic shifts were canceled out by a nearly equal rightward trend among white voters — a trend largely absent in other battleground states. The state’s white voters are far older and more Southern than in the other battlegrounds. The result: Florida barely budged, even as some Northern states with fewer demographic changes moved leftward — states like New Hampshire, Iowa, Wisconsin, Ohio, as well as Colorado.

The Republicans might not be able to keep up with Florida’s demographic shift any longer. The early polls show Mrs. Clinton with a consistent advantage.

Here’s the unsurprising reason: Mr. Trump has alienated Hispanic voters, making the last decade of demographic shifts even more potent. According to The Upshot’s estimates, Mr. Trump is losing among Hispanic voters in Florida by a 30-point margin, up from Mr. Romney’s 22-point deficit in similar estimates of 2012. (The estimates are based on a combination of pre-election polling, election results and demographic data.)

The danger for Mr. Trump among Hispanic voters could be greater in Florida than anywhere else. Not only are there more Hispanic voters than in any of the other big battleground states, but the Hispanic vote has also been unusually Republican. Put it together, and 6 percent of Florida’s voters in 2012 were Hispanics who supported Mr. Romney. There’s a lot of room for the G.O.P. to fall if it loses a big chunk of those voters.

The more surprising part of Mr. Trump’s problem is the white vote. He’s winning the state’s white voters by 19 points — slightly below Mr. Romney’s 20-point edge in similar estimates from 2012, even though the state’s white vote is not especially well educated (Trump has generally been performing well nationally among whites without college degrees).

Why is Mr. Trump struggling with white Floridians? Mrs. Clinton appears to be running ahead of Mr. Obama among older and Southern white voters, reversing the one trend that has helped Republicans over the last decade.

This might have been a problem for the Republicans even if Mr. Trump and Mrs. Clinton were not the nominees. The G.O.P.’s gains among white voters in the state, particularly in the Florida Panhandle, appeared to slow in 2012 — allowing Mr. Obama to hold a state he carried by just three points in 2008.

Mrs. Clinton might also be a particularly good fit for Florida. She won the Florida Democratic primary decisively in both 2008 and again in 2016. Florida is the only state that I’m aware of where Mrs. Clinton has often done better than Bernie Sanders in pre-election head-to-head polls versus Mr. Trump. The same thing was true in the spring of 2008, when Mrs. Clinton led in Florida polls against John McCain while Mr. Obama trailed him.

It’s hard to overstate how important it would be if Florida shifted to the left. Mrs. Clinton could lose all the other swing states where she’s airing ads — Ohio, Nevada, Colorado, Virginia, North Carolina, Iowa and New Hampshire — and still win the presidency if she won Florida and if the rest of the map held to form. She would even be favored to survive a loss in Pennsylvania, perhaps the only big blue state where Mr. Trump has a realistic chance to win. (She would need only two of the other battleground states, like Nevada and Virginia, to compensate.) Without Florida, the Republican path to the presidency gets very rocky.