Mr. Jones has adopted a different playbook. While he has voted with Mr. Trump half of the time, according to the website FiveThirtyEight, more than nearly any other Senate Democrat, he has stood with national Democrats on high-profile issues, including opposing Justice Kavanaugh. He used his first speech on the Senate floor to call for new gun control measures, tackling an issue that has long been anathema to many Southern Democrats.

Democrats hope voters will value Mr. Jones’s decision to stick with his gut when it comes to difficult political issues, even if that means siding with the national party.

“He’s a guy who’s going to be true to himself,” said John Anzalone, a Birmingham-based Democratic pollster. “Authenticity is what sells in a place like Alabama.”

But there’s also little question that Mr. Jones hit the political jackpot with his special election in 2017 by facing Roy S. Moore, an already-controversial Republican opponent who became all but radioactive after charges of sexual assault against underage girls. Even with that baggage, Mr. Jones beat Mr. Moore by only about 20,000 votes.

A number of Republican officials in the state, including Representative Bradley Byrne and the Alabama State Senate president pro tempore, Del Marsh, are considering challenging Mr. Jones next year. None have the liabilities of Mr. Moore, who motivated a surge of black Democratic voters while prompting some Alabama Republicans to cross party lines and others to stay home from the polls.

On top of those local challenges, Mr. Jones will also be running against the backdrop of the 2020 presidential election, a contest that will both highlight the national Democratic platform and bring out conservative voters eager to support the president.

Even some of his supporters don’t expect to be represented by Mr. Jones for much longer.

“Senator Jones, bless his heart, he’ll be a one-term senator,” said Sheila Pressnell, 61, as she walked through a Huntsville shopping center popular with employees of NASA and other government agencies. “The only reason he got it was because he was up against a child predator.”