“It feels different,” Debbie Mucarsel-Powell, the Democrat running against Mr. Curbelo, said over a plate of ceviche in a campaign office above a sushi-Thai restaurant at a strip mall. “Gillum, maybe, is getting people more excited.”

This much enthusiasm is a departure. In past years, the party has struggled to raise money and field credible candidates in many districts. So thin is the Democratic bench, according to Mr. Buckhorn, the Tampa mayor, that even staffing a Gillum administration could be challenging.

“We’ve been out in the wilderness for so long that many of the folks who normally would step in are too old, out of politics or doing something else,” he said.

This is a challenge Democrats would be happy to have. The day after Mr. Obama’s visit, the party’s candidates gathered in West Palm Beach to appear with a more overtly Floridian campaign closer: Jimmy Buffett, summoned to preach peace, love and shakers of salt.

But first came the undercard. Mr. Nelson strode onstage, looking out on a lawn of beach towels and avian-casual button-downs, and promised to be quick.

“Rather than giving a speech,” he said, “I’m going to introduce Andrew Gillum.”