Since March, the U.S. government has sent Venezuelans and Salvadorans accused of being affiliated with the Tren de Aragua and MS-13 gangs to El Salvador, where Mr. Bukele agreed to hold convicted criminals for the United States, for a fee.

Calif., is leading a group of congressional Democrats to El Salvador to secure the release of Kilmar Abrego Garcia, the mistakenly deported Maryland man.