Background: The new map divided one district into four, each with a much smaller Black population.

At issue in the ruling was an area previously mapped as House District 5, which stretched from Jacksonville to Tallahassee along Florida’s northern border with Georgia.

The district, whose voting population was about 46 percent Black, had elected Al Lawson, a Black Democrat, in the 2016, 2018 and 2020 elections. Its voting patterns were racially polarized, with Black residents mostly voting for Democrats and white residents mostly voting for Republicans.

In the new map approved by the Florida Legislature and signed by Gov. Ron DeSantis before last year’s midterm elections, that area was divided into four districts whose voting populations ranged from about 13 percent to about 32 percent Black. In 2022, all four districts elected a white Republican, one of whom defeated Mr. Lawson in the process.

“Under the enacted plan in 2022, North Florida did not elect a Black member of Congress for the first time since 1990,” Judge Marsh wrote in his ruling, in a list of facts that weren’t disputed by either side.

Why It Matters: The redrawn maps could have a national political impact.

In addition to having a major effect on Black voters’ representation in Congress, redistricting rulings like this one could significantly affect the national political landscape.

Given the closely divided House of Representatives, these decisions can mean the difference between a Republican or Democratic majority without a single voter changing sides. Which party controls Congress after the 2024 elections may depend on legal challenges to maps in Florida and several other states, including Alabama, Louisiana, New York, North Carolina and Wisconsin.

What’s Next: Pending an appeal, Florida may have to draw a new map for 2024.

Under the ruling, Florida is forbidden to use the unconstitutional map in the 2024 election, and state legislators are required to draw a new map that does not diminish Black Floridians’ voting power.

But this was a lower-court ruling, and Florida officials can appeal. The case could end up before the Florida Supreme Court, which is controlled by appointees of Mr. DeSantis and could reverse the ruling.

If legislators do have to redraw the map, it remains to be seen what the new version will look like — and whether legislators will comply with the ruling or seek to test its boundaries to maximize Republican advantage, as legislators in Alabama did after the Supreme Court ruled this year that their map violated the federal Voting Rights Act.