It makes Castro, whose twin brother Julián Castro is running for president, the latest in a line of well-known Democrats around the country who have resisted prodding from party allies to jump on the ballot and help efforts to reclaim a Senate majority next year.
It also sharpens the field into focus for Republican incumbent John Cornyn, who even without Castro may still face his first serious re-election challenge since joining the Senate in 2002. Air Force veteran MJ Hegar, who nearly won a House seat last year and became an overnight Democratic sensation over her viral campaign ads, announced a Senate run last week.
Castro announced his decision in an interview with Hearst Newspapers. "If and when I run for another office, it is likely to be something that takes me back home to Texas," he said. Texas hasn't sent a Democrat to the Senate since the 1970s. But former Rep. Beto O'Rourke's near-victory over Republican Sen. Ted Cruz last year revealed a shifting political landscape in the nation's biggest conservative state that Democrats are eager to newly test, even though the 2020 Senate map remains an uphill climb. O'Rourke now is running for president.
Democrats need to flip as many as four seats next year to claim a Senate majority. But they've been spurned by big names in states where incumbent Republicans are seen as potentially vulnerable. Stacey Abrams, who narrowly lost the Georgia governor's race, said Tuesday she would not run for Senate next year. And in Colorado, former Gov. John Hickenlooper is running for president after resisting calls to challenge GOP Sen. Cory Gardner.
Texas isn't considered as shaky for Republicans as elsewhere. Cornyn is a formidable incumbent who is already sitting on $7 million in campaign funds, a bigger stash than of any GOP senator. Castro, 44, has been one of the Democratic Party's brightest stars in Texas for a decade, and there was a sense his time had come after O'Rourke chose to focus on the White House rather another Senate bid.
But after a Senate campaign appeared almost inevitable just a month ago, Castro went quiet in recent weeks, and his apparent indecision grew to frustrate even fellow Democrats. It's the second time in two years that Castro has rejected a Senate run. The four-term congressman also flirted with challenging Cruz but decided around this same time in 2017 to stay in the House. That cleared a path for O'Rourke, who went on to only lose by less than 3 percentage points to Cruz — a nail-biter in a state where Republicans haven't lost a statewide race in 25 years.
This time around, Castro was urged by Texas Democrats and Latino groups to pick up where O'Rourke left off, as supporters saw him as a high-profile candidate who could inspire Latino voters in a state with a booming Hispanic population. Julián Castro also stoked his brother's possible Senate candidacy while out on the road for president.
But there were questions about whether Senate Democrats had a preferred candidate in mind for Texas. Asked last month whether Senate Minority Leader Chuck Schumer was encouraging him to run, Castro said he had a "good meeting" with Schumer but did not elaborate. Schumer had also met with O'Rourke and Hegar.
"Maybe on second thought he's decided to stand down and just to accede to Mr. Schumer's hand-selected candidate," Cornyn said Wednesday, before Castro made his announcement. Castro had said a contested primary didn't worry him. But challenging Hegar would have nonetheless set off a major Democratic nomination battle, which has become a rarity in Texas as the party often struggled to get any Democrats to run in the face of GOP dominance.
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