Joaquín Castro, a 42-year-old third-term Democratic congressman from San Antonio, who with his twin brother, Julián, has for years carried the hopes and dreams of Texas Democrats, announced Monday that he will not run for the U.S. Senate seat held by Republican Ted Cruz in what would have been a nationally watched 2018 race.

But Castro’s decision was not unexpected, and Democratic disappointment was cushioned by the candidacy of Beto O’Rourke, a 44-year-old third-term Democratic congressman from El Paso with an underdog appeal, who has been criss-crossing the state to enthusiastic crowds for months.

Castro had also traveled the state to large welcomes, testing sentiment in the early months of the Trump presidency, and in an email to supporters Monday, he said, "In every conversation, one thing was clear: Texans want a senator who will stand up for their families, not stand down to Donald Trump.

"I’ve kept my pledge to fight for hard-working Texans, and I’ll keep doing that," Castro continued. "However, with the threats posed by Russia and North Korea, coupled with the reckless behavior of this administration and their failure to invest in economic opportunity for the American people, at this time I believe I can best continue that work by focusing on my duties in the House Foreign Affairs and Intelligence committees."

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"I think, in the end, this is probably a really smart choice," said Christian Archer, a San Antonio political consultant with deep ties to the Castro family. "People across the country see Joaquín Castro as a rising star who can wait a decade and still be a young guy running for a higher office."

Longtime Democratic consultant George Shipley echoed Archer’s sentiments.

"He did the right thing and made the right decision for the country," said Shipley, who is friends with both Castro and O’Rourke. "He put his job ahead of his personal ambitions."

Texas Democratic Party Chairman Gilberto Hinojosa said that he believed Castro would have been "a formidable candidate against Mr. Cruz" — and views him as someone who will run and win statewide office in Texas at some point.

"We know he would have been a great candidate," Hinojosa said. But he said, in O’Rourke, "we have a great candidate right now who’s out there who is drawing huge crowds all over the state of Texas," including parts of Texas where Democrats don’t tend to draw big crowds.

While a Castro-O’Rourke battle might have been invigorating for Texas Democrats, it would have run the risk of spending precious resources, leaving a reservoir of bad feelings and costing the party two of its brightest stars in Congress. O’Rourke has less to lose because he had already promised not to serve more than four terms in the House.

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"Joaquín is an extraordinary member of Congress, someone who has led a principled, effective fight for his constituents and our country on the issues that matter most," O’Rourke said. "I had a chance to speak to him today and told him that I understand his decision and wish him the best going forward. I know that we will continue to work together for a long time to come, and I am grateful for that."

Castro’s mother, Rosie Castro, is a legendary political activist from San Antonio. Julián Castro was mayor of San Antonio before being appointed secretary of Housing and Urban Development by then-President Barack Obama.

Julián Castro is writing a book and said he will not run for office in 2018.

Joaquín Castro first won election to the state House in 2002 and represented the city there for a decade before running to replace retiring Democratic U.S. Rep. Charlie Gonzalez in 2012.