In this way, he’s representative of many Latino voters. Only 13 percent of Latinos who are currently registered to vote in the United States speak Spanish as their primary language, according to the Pew Research Center.

So, then, who is all this campaign-trail Spanish for?

More than 40 million people in the United States speak Spanish, making it the second-most spoken language in the country behind English. But the history of Spanish in the United States, and who feels comfortable speaking it publicly, is complicated. Mr. Castro’s biography illustrates some of the complexity.

As a child, he often accompanied his mother, a prominent Chicana activist, to political meetings and protests. She told him stories of how she had been shamed and forcefully told not to speak Spanish. While he heard the language on television and from his grandparents, he rarely spoke it. When Mr. Castro was elected to the San Antonio City Council, he sought out a private tutor.

“There is an irony,” he said in an interview. “There’s a greater expectation, because I’m Latino, of speaking Spanish. Many folks outside of second or third generation Latino communities are not aware of the history of the attempts to eradicate the Spanish language from families.”

Even as the Latino population in the United States continues to grow, and a majority of those who are parents now speak Spanish to their children, Latinos tell pollsters that they don’t view Spanish as essential to the culture.

About 28 percent say Spanish skills are a requirement for someone who identifies as Latino, according to Pew. A recent poll by UnidosUS, a Latino advocacy group, showed that a candidate’s ability to speak Spanish was last on a list of Latino voters’ priorities, well below “values diversity” and “willing to work with both parties.”