DeLAND, Fla. — Interstate 4 slices through Florida's midsection, separating densely populated, boisterous South Florida from the state's more rural and conservative north.
But this asphalt artery that stretches 132 miles from Tampa east to Daytona Beach also passes through some of Florida's most coveted electoral turf, serving as the Sunshine State's main battleground for presidential hopefuls.
"We kind of laughingly call it the highway of heaven for the candidates, because if they win I-4, they win Florida," said Susan MacManus, a political science professor at the University of South Florida in Tampa. "The I-4 corridor is the new growth part of the state, and the most politically competitive part of the state."
More than 40 percent of Florida's registered voters live in counties straddling the interstate, which includes the burgeoning Orlando area and popular retirement centers like Lakeland and Winter Haven, while in between are millions of acres of citrus groves and scrub pine woods.
Slightly more than half the corridor's voters are Republican. But about 20 percent of the electorate are registered as independents or with minor parties, making the area highly desirable for candidates, and deeply frustrating for pollsters.
"I always think of Central Florida — the I-4 corridor — as the tipping point of the state," said Aubrey Jewitt, a political-science professor at the University of Central Florida in Orlando. "It's an area of swing voters."
Between 45 percent and 50 percent of Florida's Republican voters live along the corridor, as do 40 percent to 45 percent of the state's Democrats, said Erin VanSickle, a spokeswoman with the Republican Party of Florida.
"If we win the I-4 media market by four or five percentage points, we win the state by four or five points," Ms. VanSickle said. "It basically mirrors what the state is doing."
Jennifer and Chris Ditslear of DeLand, a quaint old Florida town of about 25,000 residents east of Orlando, represent the corridor's divided political alliances; she's a Democrat, while her husband is a Republican.
"DeLand is a microcosm of Florida and the Florida electorate — always has been," said Mr. Ditslear, an attorney. "We're an equally divided town for sure."
Orlando, Florida's largest inland city, is a key prize for any candidate with an eye on winning the state. Sen. John Kerry of Massachusetts, the 2004 Democratic presidential nominee, carried the county by fewer than 1,000 votes over President Bush four years ago.
Orlando area's Democratic electorate tends to be more diverse, younger and less predictable than its Republican voters, who are generally moderate but less likely to cross party lines during elections, political analysts say.
"In general and statewide elections, it usually falls on the quality of the Democratic candidate as to how competitive the race will be in Central Florida," Mr. Jewitt said.
The Orlando area's sizable Puerto Rican community leans Democratic but to a much lesser degree than other Hispanic voters in the state, particularly South Florida's staunchly Republican Cuban-Americans.
Republican former Gov. Jeb Bush, who stepped down last year after two terms and who speaks fluent Spanish, received considerable support from the Orlando Puerto Rican community. And Republican Sen. Mel Martinez, who was born in Cuba and raised in the Orlando area, also is popular among Central Florida Hispanic voters.
"Puerto Ricans in Orlando are classic swing voters," Mr. Jewitt said.
Central Florida's booming hotel and tourist industries also contribute to the area's highly transient and unpredictable electorate. Many service workers are Hispanic, low paid, and are notoriously difficult to organize and mobilize during elections, say Democratic officials who see them as part of their coalition.
"The majority of voters in my county are Democrats, but Republicans often have an easier time organizing and often have better turnout because of the transient nature of the Democrats," said Michael Harford, vice chairman of the Democratic Party's Executive Committee of Osceola County. "But I've seen the area go [Democratic] blue in national elections and then vote Republican in local races."
Republican presidential candidates often find success in Hillsborough County, home to Tampa, even though Democrats hold a slight advantage in numbers over Republicans here. Mr. Bush easily carried the county in 2000 and 2004.
The city's conservative base is buoyed by the many current and former military personnel living in the area, which includes MacDill Air Force Base.
"This is very much the anchor of the I-4 corridor" during statewide and national elections, said Jim Browne, executive director of the Hillsborough County Republican Party.
The Tampa Bay area media market, which includes several counties, also is the most expensive in the state for political advertisements.
"All the major [Republican] candidates have been here recently, or will be here this week," Mr. Browne said last week. "You have to be a truly dynamic candidate to win the state of Florida."