Steve Cohen

Representative Steve Cohen, the lone remaining Tennessee Democrat in Congress, announced on Friday that he would not seek re-election after the Republican-led General Assembly carved up his Memphis-area district.

The announcement is the first indication of how the dilution of Black Democratic voting strength will scramble political fortunes after the Supreme Court weakened the Voting Rights Act last month. Tennessee was the first state to draft and approve a new map, slicing Mr. Cohen’s majority-Black seat in and around Memphis into three new districts.

The redistricting rush is also hastening a generational shift that had already been roiling the Democratic Party. Mr. Cohen, 76, had already been in the middle of an acrimonious primary contest with State Representative Justin J. Pearson, 31, a Black Democrat who had skyrocketed to national attention after he was temporarily expelled from the state legislature over a gun control protest in 2023.

The incumbent had resisted calls to step aside for a younger successor and has repeatedly dismissed Mr. Pearson’s more fiery political approach. Mr. Pearson announced earlier this week that he would still run for Congress in the now-heavily Republican Ninth Congressional District, and a few other Democrats signaled interest in running in one of the new seats.