New Yorkers mostly know it as being the home of the Alamo. Sometimes they’ll say to me, “Oh, isn’t there a river there?” To which, I usually joke that, yes, as a matter of fact, it is the Venice of Texas. (It is nothing like Venice.) Even when the Spurs racked up national N.B.A. championships, the coastal sports media dismissed them as boring. (Luckily, another local hero, Coach Gregg Popovich, is around to belittle said national media.)

“We’re not Houston or Dallas, we don’t boast,” said Leticia Van de Putte, a San Antonio native and former state senator who ran for lieutenant governor of Texas in 2014. “We never give ourselves the ‘atta boys’ or ‘atta girls,’ it’s not in our nature.”

If you spend enough time in San Antonio, it’s hard not to run into one of the Castro brothers. (“Half the time they think I’m my brother,” Julián Castro said). When we were both back home, Mr. Castro and I usually caught up over tacos, so, just before Christmas, when he was in New York, I met him at La Esquina, a taqueria in Midtown.

“Not as good as Texas, but not bad,” Mr. Castro said, a cautious politician’s verdict on the $4.99 barbacoa tacos that we both knew would have been fluffier and cost 99 cents in San Antonio.

We spent the first few minutes catching up on local gossip — a mutual friend; The San Antonio Express-News holding a grudge because he didn’t give it an exclusive about his plans to run. (“They put my story in the Metro section!”)

When asked by reporters why he was running, given that he hardly registers in the national polls, Mr. Castro talked about his hometown. “I said, ‘Go to my neighborhood that I grew up in — nobody was the front-runner there,’” he recalled. Mr. Castro will base his campaign in San Antonio, starting with a kickoff rally next Saturday.

Until recently, San Antonio was reliably red, but like much of the Southwest, it has been transformed into a battleground by its young and Latino population, bad news for Republicans fearing an increasingly purple Texas. In the midterms, every major statewide candidate, including Senator Ted Cruz and his Democratic opponent, Beto O’Rourke, poured resources into winning San Antonio.