Those rallying behind Gillum in recent days include 16-year-old Sari Kaufman, a Parkland survivor who spent October 21 canvassing for the Democratic gubernatorial candidate.
But Republicans pooh-poohed the former vice-president's appearance for Nelson and Gillum, saying he was "wasting his time". His Democratic challenger Gillum has been critical of the current White House.
Voters will decide between Gillum and DeSantis on November 6.
In an interview, Sari suggested young people are more excited about Gillum than Nelson, particularly because of Gillum's status as a younger candidate running statewide for the first time.
In Tampa, Biden said he had never met another senator with more "character, courage, integrity and decency" than Democratic U.S. Sen. "And he performs as all weak people do", Gillum said.
Several recent national polls - including surveys from CNN, the Washington Post and ABC News, and the Wall Street Journal and NBC News - have also found wider Democratic leads in generic congressional ballots among likely voters than among registered voters, and have found signs of strong enthusiasm among Democratic voters.
Gillum was striking a more measured tone that was aimed at inclusion.
DeSantis also said that he would work with Trump if he were elected. "We have to get out there and vote like our lives depend on it - because our lives do depend on it". At one point he called DeSantis Trump's acolyte. Bill Nelson at 50% to Republican Gov. Rick Scott's 45%.
Gillum's speech was geared toward helping college students out of debt, creating affordable housing and securing jobs, so students can comfortably live in Florida after graduating.
"He scares me, I'm sorry", retiree Suzanne Zimmerman, a member of Villagers for Trump, said of Gillum. "He's presided over a crime-ridden city", DeSantis said.
Body double used in attempted cover up of murder of Jamal Khashoggi
For weeks, the Saudi government denied that it killed Khashoggi and said he walked out of the consulate after his October 2 visit. Lubna Olayan, a Saudi businesswoman moderating the forum's first session Tuesday, opened with remarks about Khashoggi.
He furthermore discussed the importance of Medicaid and curtailing global warming. You have more water; you have flooding. This election is bigger than politics.
"This is all about a president who's trying to amass power so he can abuse power", Biden said, as people periodically encouraged him to run for president by shouting "2020!"
"We all have friends that sometimes let us down", Gillum said of Corey, while also pushing back at DeSantis with a suggestion this, too, was a racial dog whistle: "I'm a hard working person", he added.
The charged crowd shouted remarks about Trump, but Biden demanded the crowd be attentive.
Biden called the upcoming midterm elections a fight for "the soul of America" and said voters have a chance to reset "the moral compass of this country".
"I am not looking for cheers, I want you to do something beyond cheers", Biden said.
DeSantis denied claims he denies the science behind the issue, saying, "I don't want to be an alarmist".
Biden continued to discuss Trump's behavior by pointing to his neglect of Puerto Rico after Hurricane Maria and the mistreatment of immigrants.
DeSantis, a former GOP congressman who resigned to focus on this contest, began the CNN debate by calling Gillum a "failed mayor" and alleging that he was "involved in corruption" before the Democrat had a turn to speak. Both describe themselves as pro-Israel: On his site, Gillum says "I believe strongly in the unique relationship between the United States and Israel" and that he "will continue to push back against anti-Israel efforts, like BDS".