MIAMI — Andrew Gillum’s campaign bus was running late, tied up in rain and traffic on Interstate 95, so his relatives waiting to catch a glimpse of the final days of his surging candidacy had a few minutes to reminisce about the politician as a young boy.
“He’s always been a kid that was different from all the rest,” George Jackson, his 71-year-old uncle, said of Mr. Gillum, the son of a bus driver and a construction laborer who grew up in the southern end of Miami-Dade County. “Always had a book up to his head. His grandmother always told him that he was special.”
Grandma, it seems, was right.
On Tuesday, Mr. Gillum, 39, became the first black nominee for Florida governor, achieving a stunning and improbable come-from-behind win over four wealthy Democratic challengers whose personal fortunes proved no match for Mr. Gillum’s compelling life story and progressive message.
“We’re going to unite this state,” Mr. Gillum, the mayor of Tallahassee, told a crowd of supporters on Tuesday night. “What’s going to bring us together is our common and shared belief that regardless of where you come from, regardless of what your mother or your father did for their profession, regardless of what side of the tracks you live on that, that every singly Floridian ought to have their equal and fair shot at the American dream.”
Mr. Gillum’s upset victory mirrors neighboring Georgia, where Stacey Abrams defied expectations by easily securing the Democratic nomination in a bid to become the nation’s first black female governor. The November governor’s race in Florida will pit Mr. Gillum, who campaigned for Hillary Clinton in 2016 and with Bernie Sanders this month, against Representative Ron DeSantis, who dominated the Republican primary by embracing the policies and style of President Trump.
Some Democrats worry Mr. Gillum heads into the general election untested. He polled so low for so long in the lead-up to the primary that none of his rivals seriously attacked him, despite apparent vulnerabilities. The F.B.I. has been investigating Tallahassee City Hall for suspected corruption, though Mr. Gillum has repeatedly said he is not the target of federal agents. And in Florida, where Big Sugar is perhaps the top foe of environmentalists and Democrats, Mr. Gillum has been more muted in criticizing the industry, aware that it employs many working-class African-Americans and Latinos who make up a key part of his base.
Yet none of his opponents could compete with Mr. Gillum, a married father of three young children, on enthusiasm from Democratic voters tired of seeing their bland, centrist candidates lose one-point elections, like Alex Sink in 2010 and Charlie Crist in 2014. In Mr. Gillum, they saw a champion of the working class who could attract young and minority voters to a Florida midterm electorate that leans conservative.
“Most of the candidates, they’re focused on negative views of Trump and to be honest, that’s not relevant to Florida,” said Bernadette Albury, 41, who met Mr. Gillum at her church in liberal Broward County last year and voted for him on the first day of early voting. “A lot of people, they’re not geared for change — and we need change.”
Mr. Gillum relied on the support of the billionaire progressive activists George Soros and Tom Steyer, whose money went to field organizing as opposed to television advertising. The strategy was a risk in as big a state as Florida, home to 10 media markets. Voters learned about Mr. Gillum by word of mouth and through highly produced web videos — including the one launching his candidacy, in which he noted each of his three older brothers had a criminal record.
By the time Mr. Gillum reached the last weekend of the campaign, his voice was hoarse and his speeches openly invoked the last African-American politician to win a statewide contest in Florida: President Barack Obama. Speaking Saturday at a Cuban-themed bar blaring reggaeton in Little Havana, Mr. Gillum pointed to the date of the primary — Aug. 28, the same day Mr. Obama accepted the Democratic presidential nomination in 2008, and the same day the Rev. Dr. Martin Luther King Jr. delivered his “I Have a Dream” speech in 1963.
“Florida has an opportunity, in our darkest days as a nation, to remind the rest of the country through this election that all humanity is not lost,” he said. “That we’re stronger when we’re together in this state. Y’all — there’s more of us!”
Standing quietly in the back of the room, unseen by the candidate and unrecognized by the crowd, Mr. Gillum’s uncle clapped louder than anyone else around him.