"We have to make sure Rick Scott wins and wins big" against the incumbent, Democrat Bill Nelson, the president said.
He also appealed for voters to elect more Republicans to Congress, saying Democrats are opposing his to-do list and slowing consideration of his Supreme Court nominee, Brett Kavanaugh.
Democrats "don't want to give Trump any victory," he said. "They will do anything they can to not help the Trump agenda."
Trump has threatened to shut down the federal government over his push to overhaul the nation's immigration system and pau for his promised wall along the U.S.-Mexico border. But officials say Trump has reassured staff in private that he will not provoke a fiscal crisis before the November election, when the GOP hopes to retain its majority.
The president avoided making an outright reference to a government shutdown during the rally, saying, "We may have to do some pretty drastic things" unless Democrats support his agenda.
He spent much of the rally highlight strong economic numbers and praising DeSantis as "a tough, brilliant cookie." He predicted DeSantis will defeat Agriculture Commissioner Adam Putnam in this month's primary.
Trump, who makes frequent trips to Florida and his private Palm Beach Mar-a-Lago club, criticized Nelson's policies and claimed the only time he sees the senator is "five months before every election."
"After a while, you forget who's the senator," Trump said.
Scott didn't join Trump at the rally but appeared with him at an earlier event.
DeSantis has tied his campaign for governor to Trump, appearing on Fox News more than 100 times to talk about federal issues and defend the president. DeSantis has campaigned with Fox's Sean Hannity and Donald Trump Jr. and uses humor in a new ad to show his alliance with the president, teaching one of his two children to "build the wall" with blocks.
Putnam, a state agriculture commissioner and former congressman, has run a more traditional campaign for governor, aiming to build upon his family's deep ties to the state.
Trump railed against the idea of allowing noncitizens to vote in some elections and said only citizens should vote. He also advocated requiring voters to present photo identification; Florida already has such a law.
"The time has come for voter ID like everything else," Trump said, before making his claim about groceries.
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