“Gillum is the great disrupter,” Mr. Stipanovich said. “He is the Donald Trump in this election. DeSantis represents more of the same. That’s not an exciting message when people are excited.”

At a recent party dinner in West Palm Beach, Mr. Gillum, the main attraction, took some time to address a packed, cheering audience. “There are more of us than there are of them,” he said. “The power of the people is greater than the people in power. And we must not sound like and act like we are Republican Lite, y’all. We have a belief system.”

The Tallahassee mayor’s ideal Florida is a vastly transformed place where immigration enforcement is less aggressive, all workers earn at least $15 an hour, the “stand your ground” self-defense law no longer exists, everyone qualifies for Medicare, ex-felons can vote and corporations pay higher taxes so that the starting salary for teachers is $50,000 a year. Much of this vision has no chance of becoming reality in Florida’s conservative legislature.

Instead, Mr. Gillum’s fate will depend on how well his aspirational vision plays in the nation’s largest, most mercurial battleground — a place of such cultural and economic diversity that it’s truly more like five states in one.

The Gold Coast, a megalopolis of Miami-Dade, Broward and Palm Beach Counties, is home to about a third of the state’s 4.9 million Democratic voters. Known for its large bloc of progressive and immigrant voters, the area also houses so many Northeastern transplants that Michael Bloomberg refers to Palm Beach County as New York City’s “sixth borough.”

South Florida Democrats, notorious for sitting out midterm elections, are expected to turn out in greater numbers thanks to increased organization efforts. After two days of early voting, more than 34,000 people have voted in deep-blue Broward County. In the last midterm, the two-day turnout was less than 14,000. So the early signs here are good for Mr. Gillum, but to win he’ll still need support among the older centrist Democrats critical to Senator Nelson’s statewide victories, which isn’t a guarantee.

Across the state’s midsection is the swing-voter-heavy “I-4 corridor,” a land of suburban sprawl, citrus groves and farms, as well as the Tampa, Orlando and Daytona Beach metro areas. Home to Disney World and Nascar’s Daytona 500, low-wage jobs supporting tourism and agribusiness predominate. If Mr. Gillum performs well there, he’ll probably have avoided unnerving too many white middle-class suburbanites while winning over workers with his messaging on higher base pay.