Why did South Texas shift towards the Republicans?

Question

The results of the 2020 US election are coming into focus, and, broadly speaking, there remains a large gap between how racial/linguistic groups vote. The Republicans ran a distinctly racist white-identitarian campaign,* and again attracted a lot of support from white working class and rural voters, and retained an overall advantage among white voters. On the other hand, Latinx and, especially, black voters heavily supported Democrats. There were some localized shifts, with, notably, many Latinx voters shifting towards the Republicans in South Florida and South Texas.

The S. Florida shift is fairly easy to account for, as the largely Cuban-American Latinx community there is skeptical of left-wing politics due to concerns about left-wing regimes in Latin America. Also, reporting before the election flagged a large amount of misinformation about Joe Biden and the Democrats as circulating in S. Florida for whatever reason (‘This is f---ing crazy’: Florida Latinos swamped by wild conspiracy theories //A flood of disinformation and deceptive claims is damaging Joe Biden in the nation’s biggest swing state.).

The S. Texas shift is more surprising, at least to me, and doesn't particularly seem to have carried over to other largely Mexican-American areas of Arizona or California. So what accounts for this shift?

_{*Response to some comments about this language: That the Republican Party under Trump has run racist campaigns is the mainstream judgement of the field of political science and of reputable media outlets, as those added links may indicate. The point of mentioning this is not to "soapbox" but to indicate a reason that a shift towards the GOP within a minority group is at least somewhat surprising or interesting.}

Answer 1

Insofar I haven't seen much beyond expert opinion on this, and most of it focused on Trump's win in Florida, where the Trump campaign advertised heavily in Spanish describing Biden as a (Venezuela-like) socialist, but for what that's worth, this was suggested about south Texas:

In South Texas, for example, Trump’s “law and order” messaging and opposition to defunding the police had some resonance in Latino communities where law enforcement, particularly the Border Patrol, is a major employer, Garcia [president of the League of United Latin American Citizens (LULAC)] said. Many residents also work in oil fields and fear Democrats’ calls to transition away from oil and gas and towards clean, renewable energy. And many are Catholic or Evangelical Christians who find Democrats’ pro-abortion rights stance abhorrent.

The Politico article from ohwilleke's answer eventually cites some experts along the same lines:

Trump’s success in the Rio Grande Valley, says Daniel Arreola, a cultural geographer and author of* Tejano South Texas*, “peels back the onion on how really conservative that Tejano ranch and small-town rural population is.”

From the brush of Laredo across more than 200 miles to the lush delta of Brownsville, there’s a legacy of a frontier culture that lives on. A place like Zapata is oil country. On weekends, the town empties out as people head into the ranchland to hunt, and nearly everyone is proudly gun-toting and God-fearing. In the deeply Catholic county, support for abortion is practically nonexistent, while support for law enforcement, the military and even Border Patrol is rock-solid.

In a parallel with what DW noted about Cuban Americans in Florida, Tejanos interviewed by Politico didn't feel targeted by Trump's negative rhetoric:

While many Trump supporters in South Texas, like Trappe, have Mexican roots and say they can understand how the president’s crude and derogatory language might seem divisive to some, few of them hear the president talking about them when he makes comments about Mexicans.

“I think when people say they don’t like Mexicans, to me it means a Mexican citizen, or a Mexican national, who has crossed illegally,” says Barrera, the Starr County Republicans chair. “And then again, not all Mexicans look alike. So when they say they don’t like Mexicans, I don’t think it means me. … It means a Mexican national who has broken the law.”

Answer 2

Politico conducted an in depth analysis of this question entitled "Trump Didn't Win The Latino Vote in Texas. He Won The Tejano Vote." According to Wikipedia:

Tejanos (Spanish: [teˈhano]; singular: Tejano/a/x; Spanish for "Texan") are the Hispanic residents of the state of Texas who are culturally descended from the original Spanish-speaking settlers of Tejas, Coahuila, and other northern Mexican states. They may be variously of Criollo Spaniard or Mestizo origin.

The gist of the analysis is that this specific community of Spanish speaking people who mostly see themselves as Mexican-Texan rather than as Hispanic or Latino, are not immigrants, mostly self-identify as "white" in race, and have a lot of broad agreement with Republican views.

The analysis is less clear about why their votes shifted so much from 2016 to 2020, but an active grassroots campaign carefully targeted to this particular community no doubt played a part.