The Democratic nominee for Senate faces strong headwinds in a decidedly Republican-leaning state. She has been a fighter, and an underdog, for years.

The first time she protested gerrymandering in Florida, Republican state lawmakers cut the video feed of her sit-in. The second time she showed up with a bullhorn. The third time, she was arrested.

“When they go low,” she says, putting a twist on Michelle Obama’s decade-old dictum, “we get loud.”

Ms. Nixon’s high-volume tactics as a state representative have made her one of the most outspoken critics of Florida’s Republican governor, Ron DeSantis. She has also at times rankled fellow Democrats. Some political veterans dismissed her campaign for the U.S. Senate as progressive whimsy — until her decisive primary victory against a more moderate opponent last month. It catapulted her onto the national stage and jolted an otherwise sleepy Democratic Party in a Republican-leaning state.

She has spent the weeks since hoarse, appearing nearly everywhere as she tries to make the case that a Black woman with a Southern drawl — who identifies as a progressive but also joined the Democratic Socialists of America in June — can win in a state President Trump has taken three times. If elected, Ms. Nixon would become Florida’s first Black senator.

National Democrats have all but written off Florida. Republicans believe the nomination of Ms. Nixon, whom they have branded a “socialist,” has only made it easier for Senator Ashley Moody, the Republican incumbent Mr. DeSantis appointed last year to replace Marco Rubio, to win in November.

But Ms. Nixon the Senate nominee, much like Ms. Nixon the state legislator, is difficult to ignore. She leans fiery while Ms. Moody leans restrained. She engages with the media and curious crowds drawn to her, while Ms. Moody mostly sticks to Fox News and friendly supporters.