Photo: Lisa Krantz, SAN ANTONIO EXPRESS-NEWS

Beto O’Rourke giveth and Beto O’Rourke taketh away.

On Wednesday, the Dallas Morning News reported that the former congressman from El Paso, who lost a close U.S. Senate race last November to Ted Cruz, will not challenge this state’s other senator, John Cornyn, in 2020.

This development has opened up a lane for U.S. Rep. Joaquin Castro, D-San Antonio, to take on Cornyn, something the Castro camp says he will “seriously consider.”

Castro flirted with a 2018 run of his own against Cruz but backed off to avoid what would have been a divisive Democratic primary contest with O’Rourke. By the most generous interpretation, you could say that O’Rourke is now returning the favor.

On ExpressNews.com: Castro, unlike O’Rourke, has much to lose in 2018

Of course, the reason O’Rourke is bowing out of the Senate race is that he appears ready to join the crowded field (13 official candidates and counting) for the 2020 Democratic presidential nomination.

And that’s where the taketh-away part comes in.

That’s because Joaquin Castro’s identical twin brother, Julián, the former San Antonio mayor and Housing and Urban Development secretary, already is out on the presidential campaign trail, making his case in the diners of Iowa and on the college campuses of Idaho. Joaquin Castro, his brother’s most loyal champion, is serving as chairman of Julián’s campaign.

In an impressive display of sibling support, Joaquin has even cultivated some facial hair in recent weeks to make it easier for voters to distinguish the Castro who’s running for president from the Castro who serves on the House Intelligence Committee.

O’Rourke’s expected entrance into the presidential sweepstakes would clog — though not necessarily block — Julián’s path to the nomination.

O’Rourke’s punk-rock, skateboarding, shoot-from-the-hip hipster appeal would dilute some of the power from Julián’s generational-change message and make the former mayor only the second-best-known 40-something Texan in the race.

The Castro brothers always have plotted their career moves as a team, making sure not to step on each other’s big moments. Joaquin put aside his congressional ambitions until Julián was safely ensconced in the mayor’s office. When Joaquin made his congressional move in 2012, Julián took a back seat.

On ExpressNews.com: Castro makes first move from West Side to West Wing

For the Castro family, this is Julián’s time.

After all, you could make a good case that Joaquin, as a four-term member of the U.S. House of Representatives, chairman of the Congressional Hispanic Caucus and vice chairman of the House Foreign Affairs Committee, has stronger credentials for a presidential bid than his brother.

But this is Julián’s time.

That fact might lead some observers to conclude that Joaquin will resist a campaign against Cornyn because it could detract from his brother’s presidential candidacy.

In truth, the opposite is the case.

A Senate campaign from Joaquin would immeasurably boost Julián’s presidential prospects.

For one thing, the national media would get swept up in the story of two twins running for high-profile offices at the same time. For another, Joaquin’s statewide presence on the 2020 Texas primary ballot would be a big turnout asset for his brother.

So if Joaquin passes on a 2020 Senate race, it won’t be because of his brother’s presidential campaign. It will be about the particulars of Joaquin’s own unique situation as a 44-year-old congressman, family man and emerging force in his party.

Joaquin has the most coveted congressional seat in South Texas, representing the only district entirely contained within the borders of Bexar County. It’s a safe Democratic seat that he could hold for the next 30 years, if he wanted.

It’s not a wild stretch to suggest that someday he could become the first Latino speaker of the House.

Already, he’s a force on Capitol Hill, as demonstrated by his sponsorship of a resolution that would block the national emergency that President Donald Trump recently declared for the funding of a border wall. The resolution, which the House passed Tuesday, provides a key test of the separation of powers between this country’s legislative and executive branches.

Challenging Cornyn would force Joaquin to give up his seat.

In addition, Cornyn makes for a more elusive adversary than Cruz. Almost willfully bland, Cornyn stirs neither the animosity nor the worship that Cruz generates, though they share a compulsive allegiance to Trump.

A Quinnipiac poll released Tuesday found that 43 percent of Texans approve of the job that Cornyn is doing, compared with 51 percent for Cruz. Only 26 percent, however, disapprove of Cornyn’s performance, compared with 40 percent for Cruz.

As Joaquin mulls his options and Julián builds support for his presidential campaign, O’Rourke prepares to make his big announcement. They make for an ever-shifting political triangle — with sharp angles.

Gilbert Garcia is a columnist covering the San Antonio and Bexar County area. Read him on our free site, mySA.com, and on our subscriber site, ExpressNews.com. | ggarcia@express-news.net | Twitter: @gilgamesh470