From Texas Standard:
Though he hasn't made an official announcement, Texas Monthly recently reported that Joaquin Castro could soon announce plans to challenge John Cornyn for his Senate seat in 2020. If Castro runs, his own seat in Congress will be open. He represents a district that's been solidly Democratic for years, and now some are speculating about who would run to replace him.
Gilbert Garcia, metro columnist for the San Antonio Express-News, has been pondering the musical-chairs game of sorts that a Castro Senate bid could set in motion.
Garcia says San Antonio comprises five congressional districts, and that the 20th is the one Democrats covet most.