WASHINGTON - U.S. Rep. Joaquin Castro has decided not to seek the Democratic nomination to challenge Sen. John Cornyn, choosing instead to continue pursuing a fast-rising career in Congress focusing on security and border issues.

Castro’s decision could pave the way for a contest in 2020 between Cornyn and Mary Jennings “MJ” Hegar, an Afghanistan war veteran who ran a strong but losing race for Congress last year and who declared her candidacy last week.

Castro, 44, of San Antonio, announced his decision to stay out of the race in an interview with Hearst Newspapers.

“Right now, I’m going to focus on my work in the House of Representatives. I’ve been doing what I feel is important and meaningful work here,” he said. “If and when I run for another office, it is likely to be something that takes me back home to Texas.”

Castro is the twin brother of Julián Castro, a former San Antonio mayor and Housing and Urban Development secretary who is running for the Democrats’ 2020 presidential nomination. The identical twins act as chief political strategist to one another, and Joaquin has the title of chairman in his brother’s quest.

His brother’s presidential campaign could have been helpful to Castro, creating excitement among Latino voters and national attention to the unprecedented effort of twins seeking high office.

But Joaquin Castro’s race also might have produced the uncomfortable scenario of extraordinarily close brothers parting ways on issues.

Joaquin Castro also had a ringside seat to his brother’s struggles to raise money, reporting a modest $1.1 million in receipts in the first three months of 2019. Thus far, Joaquin Castro has paid little attention to his own fundraising, bringing in just $36,000 in the first quarter, his Federal Election Commission report shows.

He said he is impressed with Hegar and others considering the race. “And like I have for many years, I’ll do everything I can to help our Democratic nominee win,” he said.

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Castro has enjoyed wide support among Texas Democrats and his candidacy had been widely anticipated by party officials and Latinos in public office pressing him to make the race.

In opting to remain in his safe House seat, Castro, a four-term House veteran, keeps alive the potential of winning a House leadership spot soon and perhaps becoming the first Latino to rise to House Speaker.

His seats on the Intelligence Committee and Foreign Affairs committees have made him a frequent cable TV guest. Earlier this year, he authored the resolution leading to a rebuke of President Donald Trump for invoking emergency powers to overrule congressional spending limits on construction of a border wall. In December, Castro also took over the chairmanship of the Congressional Hispanic Caucus, an alliance that has become influential on immigration and border issues.

Castro’s announcement clears a significant hurdle for Hegar, who came close to unseating veteran GOP Rep. John Carter in November and who reportedly has been urged by Senate Democratic leaders to run.

Cornyn campaign manager John Jackson issued a statement contending that Hegar is running at the behest of Senate Minority Leader Chuck Schumer, D-N.Y.

“Hegar may have the support of the New York and California elites but she has none in Texas. Shame on Chuck Schumer and DC Democrats for forcing a high-profile Hispanic leader out of the Senate race,” the statement read.

Hegar issued a statement saying that she “tremendous respect for Congressman Castro and his commitment to public service, and I know Texas will continue to benefit from his strong leadership. I am laser-focused on our shared goal of defeating Senator Cornyn next November. Texans deserve a new Senator who reflects our values of strength, courage, and independence — someone who will always do what’s right for our state and put Texas first."

Democrats are vying to challenge Cornyn, 67, a three-term senator who rose to the No. 2 post in the Senate GOP leadership and twice has led the Republican National Senatorial Committee efforts to elect members of his party.

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Cornyn has prepared for the 2020 contest, maneuvering to head off primary opposition and building a $7 million-plus campaign fund. Yet polls have suggested that Cornyn lacks the support and name recognition that might be expected of a politician who has held statewide office since becoming attorney general in 1998.