On a recent Saturday night at Ego’s, a popular Austin karaoke bar, the room went quiet as two young women stepped onto the stage. The lyrics to Ella Langley’s “Choosin’ Texas” flashed across the screen, but few in the crowd were looking. They knew the words by heart. At the song’s chorus, the room erupted into song.

Days earlier, the Pinnacle, a Nashville live music venue, was packed with country music’s biggest behind-the-scenes figures. They applauded Ms. Langley, who had won another music award, while someone performed a cover of “Choosin’ Texas.” Ms. Langley shared her gratitude remotely. Nashville is singing along to a song about choosing Texas.

“Choosin’ Texas” has spent 20 weeks at No. 1 on the Billboard Hot 100, claiming the longest run ever atop the chart for a non-holiday song. Ms. Langley, who is not from Texas, tapped into the mythology of the Lone Star State and an age-old rivalry with Tennessee. The song arrives as Southern culture spreads far beyond the region.

Texas has long insisted it isn’t really Southern, yet its image has emerged at the forefront of American culture. “The key to its identity is branding,” said Glen Sample Ely, a Texas historian. In Nashville, it was clear Ms. Langley had a bona fide country hit.