DAYTONA BEACH — When the first, sleepy stretch of Interstate 4 opened between Plant City and Lakeland in 1959, no one dreamed that this asphalt ribbon would one day become the ultimate political barometer.

For decades, political scientists, journalists and politicians have studied the now-132.3-mile swath of Florida between St. Petersburg and Daytona Beach, weighing the ever-changing impulses and vagaries of swing voters in America's most fought-over battleground.

PHOTO GALLERY: View images of Donald Trump and Hillary Clinton's visits to the I-4 corridor

The obsession continues as another neck-and-neck presidential race rolls to a close Tuesday, and the reason is simple. The so-called I-4 corridor — once Florida's "golden girdle" — decides Sunshine State elections.

"This is America's corridor of power, paved with presidents made and contenders broken," said Adam Goodman, a Tampa-based Republican consultant.

The basic political math for winning statewide races has been consistent for decades: Republicans rack up votes in conservative north and Southwest Florida; Democrats clean up in the population centers of South Florida; and the two sides battle it out in the high-growth areas along I-4.

That rural, urban and suburban stretch of central Florida extending from the Gulf to the Atlantic is neither reliably Republican red nor Democratic blue, but swing-voter purple. Democrats don't even need to win I-4, but if they keep it close they probably win Florida, and Hillary Clinton is the next president. The calculation for Donald Trump is more urgent: Without Florida's 29 electoral votes, it is virtually impossible for him to win the presidency.

So it's no accident that week after week the top two U.S. markets for presidential TV advertising dollars have been Orlando and Tampa Bay, some $60 million this year.

What's remarkable is that even after dramatic demographic shifts, especially a soaring population of Democratic-leaning Puerto Ricans in the Orlando area, I-4 remains as competitive as ever.

The explanation is an overlooked trend along the I-4 corridor and why Trump stands to win this diverse, mega-battleground state: White flight from the Democratic Party to the GOP.

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Understanding the I-4 corridor's bellwether mojo requires defining the area. It bisects six counties — Hillsborough, Polk, Osceola, Orange, Seminole and Volusia — but is far bigger than the interstate map.

"The reason it is still so competitive is because the I-4 corridor is not just Hillsborough and Orange counties," said veteran Republican consultant Randy Enwright of Tallahassee. "The Villages are part of it, and Manatee and Sarasota. … There are a whole lot of counties that we do pretty darn well in."

Politically speaking, I-4 encompasses the TV media markets of Tampa Bay and Orlando. That's 19 counties, more than 5.5 million registered voters, including 2 million Democrats, 2 million Republicans and 1.5 million Floridians registered to neither major party. Since the 2000 election cemented Florida's status as the ultimate swing state, the number of I-4 voters has jumped 52 percent and the number of independent voters 138 percent.

If it were a state, I-4 would be the size of Virginia. Winning Tampa Bay alone is akin to winning Arizona.

Altogether, I-4 accounts for 43 percent of Florida's voters.

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Orlando-Daytona Beach and Tampa Bay share a common interstate, and both are electoral swing regions. But the Orlando and Tampa Bay media markets have distinct political personalities.

Tampa Bay, home to one in four Florida voters, is a mecca for actual swing voters. That increasingly rare breed of Floridian who doesn't necessarily vote straight party line is especially prevalent in the heart of Tampa Bay — Pinellas, Hillsborough and Pasco counties.

If Tampa Bay can largely be colored bipartisan purple, today's Orlando is more like deep blue surrounded by deep red.

Not so long ago, strategists in both parties looked at the exploding population of Puerto Ricans around Orlando — 400,000 and climbing — and saw the prospect of Florida turning into a solidly Democratic state, much as California did in the 1990s.

"I am really excited, because this will be my first time voting, and I do think Hillary Clinton will be an excellent, excellent president," said Sherilyn Garcia, a 36-year-old state employee who moved to Orange County from Puerto Rico in July.

How much has the area changed politically? Consider that in Florida's virtually tied election of 2000, Republican nominee George W. Bush won Orange, Osceola and Seminole counties by about 8,900 votes. In 2012, Barack Obama won that same area by more than 98,000 votes.

But then look slightly west in the Orlando media market to the more rural Marion, Sumter and Lake counties that Bush won by a combined 26,000 votes in 2000. Thanks mainly to massive growth in the Villages, Mitt Romney won those same three counties by more than 76,000 votes in 2012.

Bill and Lora Shepherd retired to the Villages from Virginia, where Bill worked in the defense industry. They support Trump because "we don't think the country will be well off after another four years of the same and that's what Hillary would bring,'' Lora Shepherd said.

Bill Shepherd says Trump "shoots from the hip sometimes, but that does not cause us any concern because we consider the opponent. We consider her corrupt."

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It's not only growth in Republican strongholds that's counteracting the growing diversity throughout the I-4 corridor. It's also that the Democratic Party has been steadily bleeding white voters to the GOP. In 2000, 70 percent of Florida Democrats identified themselves as white. Today? 50 percent.

"A lot of it is just my party doing terribly with whites," said Democratic strategist Steve Schale, who ran Obama's Florida campaign in 2008. "Places like Daytona, which used to lean blue, now lean red. Places like Brevard, which 15, 20 years ago used to send Democrats to the Legislature, are now rock-ribbed Republican."

When Clinton campaigned in Volusia County last weekend, she went to a largely black neighborhood and to a homecoming game at the historically black Bethune-Cookman University. Not far away, vendors at the Daytona Beach Flea and Farmers market hawked anti-Clinton and pro-Trump T-shirts.

"The economy's been pretty rough for a lot of people around here," said 40-year-old James Temple, a Trump supporter selling $3 fake dog poop. "I've seen a lot of people lose their businesses. Obama has been okay, but Hillary is just not honest."

Another nearby merchant, Patricia Goode, 55, of Port Orange, was undecided. Trump scares her, but most of her friends support him.

"If Hillary just hadn't done her stupid email thing, I'd be fine with her," she said.

In Tampa Bay, the trend of working-class, white voters shifting to the GOP has helped formerly Democratic counties such as Hernando and Pasco turn Republican. It's why Democrats worry that Democratic-leaning Pinellas County (80 percent white compared to 64 percent statewide) — could be particularly strong for Trump this year, along with Volusia (78 percent white) at the opposite end of I-4.

Obama took 42 percent of Florida's white vote in 2008, when he won Florida by nearly 3 percentage points. And he took 39 percent in 2012 when he won by nearly 1 percentage point, exit polls showed. Unsuccessful Democratic gubernatorial nominee Charlie Crist won just 37 percent of the white vote in 2014, and recent Florida polls indicate Clinton could draw as little as 35 percent.

Consider that Obama would have lost Florida in 2012 if he had done 1 percent worse with white voters — even if he won 100 percent of the black vote, instead of 95 percent. Democrats hailed Obama for winning 60 percent of Florida's Hispanic vote, but he would have had to boost his performance with Hispanic voters by 4 percentage points to compensate for doing one point worse with white voters.

"If Hillary Clinton were getting the same share of the white vote today that Barack Obama got in '08, she'd be up 6 or 7 points in Florida," said Schale.

Brett Doster, a Republican strategist, sees Trump potentially appealing more to working-class, white voters than recent Republican nominees.

"Mitt Romney was such a larger than life rich guy there may have been some connectivity issues,'' Doster said, suggesting that Trump versus Clinton is less about class warfare than insider versus outsider. And that clearly benefits Trump.

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Think of I-4's electorate as a water balloon. Squeeze in one part of it, and another part expands.

Obama in 2012, for instance, netted 10,038 more votes from Pinellas County than Al Gore did when he won it in 2000. But just across the Sunshine Skyway Bridge in conservative Manatee County, Romney netted 10,327 more votes in winning there in 2012 than George W. Bush did in 2000.

By many estimates, the single-most reliable bellwether county in Florida is Hillsborough, which has voted for the winning presidential candidate in every election since 1960, except 1992 when it failed to go with a another candidate named Clinton.

In 2000, George W. Bush won Hillsborough — the most-populous county on I-4 — by more than 11,000 votes. Twelve years later, Obama won it by more than 36,000.

More than 40 percent of Hillsborough's electorate is non-white, and the growing Hispanic population is far broader than the Cubans long associated with Tampa. Among the nearly 400,000 Hillsborough residents who identified themselves as Hispanic in a 2015 Census survey, 80,000 said their family originated in Mexico, 96,000 from Puerto Rico, 94,000 from Cuba, 20,000 from Central America and 40,000 from South America.

A strong majority of these Hispanic voters lean Democratic. That and Hillsborough's growing black electorate are why Florida's ultimate bellwether county is on its way to becoming a Democratic stronghold. What's more, as the Hispanic vote grows in eastern Hillsborough and western Osceola, it may not be long before Republican-leaning Polk County becomes a new I-4 battleground county.

Ultimately, what makes the I-4 corridor such impeccable territory for picking presidents is that it is America: Midwestern, Northeastern, Southern. Country, city, suburban. Black, white, Hispanic. Native, transplant, agricultural, high-tech. Young, old, mostly middle class.

And constantly changing.

Times researchers Connie Humburg, Caryn Baird, Carolyn Edds and John Martin and Times/Herald staff writer Mary Ellen Klas contributed to this report. Contact Adam C. Smith at asmith@tampabay.com. Follow @adamsmithtimes.