When the U.S. Supreme Court rejected Alabama’s congressional map last year as an illegal dilution of Black voting power, the decision set in motion a heated redistricting battle.

Now, voters on Tuesday will head to the polls for the first time in a newly reshaped Second Congressional District, which was redrawn to give Black voters a fair opportunity to elect a representative of their choice.

The shake-up has drawn a field of nearly two dozen candidates, underscoring the rare political opportunity on offer: a primary without an incumbent, and because Black voters historically favor Democrats, a suddenly competitive race in ruby-red Alabama.

“If not now, I think I’ll be 60 before something else comes up,” said one of the Democratic candidates, State Representative Jeremy Gray, 38, as he stood near the waterworks in Prichard, Ala., trying to catch voters on their way to pay their monthly water bills.

The race is also a test of what fair representation means in a state that has repeatedly provoked federal intervention by disregarding civil and voting rights laws. On top of persuading people to come out and vote, Mr. Gray said, “it’s a lot of education when it comes to what actually happened.”