Gillum, the Mayor of Tallahassee, will become the first black candidate of a major party to run for governor in the Sunshine State.
FILE PHOTO: Republican Florida governor candidate Ron DeSantis speaks as U.S. President Donald Trump stands during a Make America Great Again Rally at the Florida State Fairgrounds in Tampa, Florida, U.S., July 31, 2018.
Sanders wrote on Twitter that Floridians had demanded "real change" with Gillum's nomination to challenge Trump-backed Rep. "That's what the political revolution is all about and Andrew Gillum is helping to lead it".
No one imagines that Gillum will have an easy time of it in Florida, a politically-volatile battleground state where Republicans won the 2010 and 2014 gubernatorial races.
Primary day in Florida and Arizona [UPDATED THROUGHOUT THE NIGHT]
Florida's Democratic primary is the most crowded since 1978, the year Bob Graham eventually won the governor's race. DeSantis entered the race in January and largely built his name recognition with near-nightly Fox News appearances.
"All indications that we are going to have an usually high turnout at least in recent years", said Michael McDonald, a political scientist at the University of Florida.
The conservative DeSantis easily won the Republican primary by highlighting his enthusiastic loyalty to Trump.
Voters in Arizona were also picking candidates for the November elections, when Democrats will try to pick up 23 seats in the U.S. House of Representatives and two seats in the Senate to gain majorities and slam the brakes on Trump's legislative agenda.
At 39, Gillum was by far the youngest candidate in the crowded field, but the most experienced in public office.
"What has made Andrew's campaign so powerful is that he's not just working hard to win an election, he has laid out a vision for a new course for the state of Florida and our country".
"We've been on this campaign trail for 18 months, and I've said numerous times that there would never be a poll showing the mayor winning this race until tonight", Andrew Gillum's communications director Geoff Burgan told ABC News' Tom Llamas and Rick Klein. "And here we are".