Rep. Joaquin Castro (D- Tex.)  (Tom Reel/ San Antonio Express-News)

Rep. Joaquin Castro (D-Tex.) will not launch a bid to unseat Sen. Ted Cruz (R-Tex.), likely clearing the field for another Democratic congressman to take on the first-term Republican incumbent and former presidential candidate.

Castro, who represents San Antonio in Congress, made his decision over the weekend and began informing close supporters and donors on Sunday, according to people familiar with his decision. The announcement caps several months of deliberations by the third-term lawmaker, whose twin brother, Julián Castro, is the former secretary of housing and urban development and is mulling a possible 2020 presidential campaign.

While Joaquin Castro says he is not considering a Senate run now, he isn't entirely ruling out such a campaign down the road. For now, Castro is preparing to run for reelection to his House seat but realizes that political developments could change his decision, said an aide familiar with the congressman's thinking who spoke on the condition of anonymity to talk frankly about political strategy.

The decision likely makes life easier for Cruz, who is unpopular back home despite his national profile and second-place finish in the GOP presidential primaries last year. Recent polls show that Cruz could be vulnerable to a well-funded challenger. A statewide Texas Lyceum poll released this month showed Castro leading Cruz, 35 percent to 31 percent. In a potential matchup between Cruz and Rep. Beto O’Rourke (D-Tex.), who announced his own Senate bid last month, the two are tied at 30 percent each.

Cruz aides dispute the Lyceum poll's findings and its sampling of Texas voters, noting that it had a larger share of Democrats than is reflective of the state's recent electoral history. With 30 percent of voters in the poll undecided, there's plenty of room for growth, Cruz aides said, adding that their own internal polling shows him in a much stronger position.

Castro, 42, won his House seat in 2012 and quickly emerged as a leading face for House Democrats, who had seen their ranks diminished over two election cycles. His plum assignments on the House Foreign Affairs and Intelligence committees are indicative of his personal interest in global affairs but also of party leaders rewarding him for several years of political and fundraising work on behalf of Democrats nationwide.

In this May 27, 2009, photo, Julián Castro, left, then the mayor-elect of San Antonio, pauses during a visit with his brother, then-state Rep. Joaquin Castro (D) in the Texas House of Representatives. (Harry Cabluck/Associated Press)

While Julián Castro earned national attention for a speech to the 2012 Democratic National Convention and later served as President Barack Obama’s secretary of housing and urban development, Joaquin Castro has devoted much more time to Texas, where he's traveled widely and cultivated party activists in urban centers and smaller cities such as Laredo, Lubbock and Midland. He has donated $1.2 million from his own campaign coffers to Texas and national Democratic candidates, aides said — party work that will continue, especially across Texas. The political travel and fundraising will also continue, especially across Texas, aides said.

By staying out of the race — at least for now — Castro avoids a potentially costly primary fight with O’Rourke, a representative from El Paso who jumped into the Senate race in March. Whichever Democrat emerges as the party nominee next year faces daunting political history: Not since Bob Bullock was reelected lieutenant governor in 1994 has a Democrat won statewide office in Texas.

Adding to the Democratic Party's challenge, statewide campaigns in Texas require a presence in 20 media markets, including three of the most expensive in the country -— meaning that a competitive race against Cruz would be a multimillion dollar affair at a time when Democrats will be defending 25 seats across the country.

“I wouldn’t count on any support from the DSCC, because they’ve got their hands full with trying to reelect 25 Democrats,” Castro said at a recent Washington Post Live event when asked about a possible Senate run, referring to the Democratic Senatorial Campaign Committee.

Castro predicted that a Senate race against Cruz would cost “tens of millions of dollars,” before adding later: “I think I could raise it, and Ted Cruz would help me raise it.”

O’Rourke told The Post in March that he’ll seek small-dollar donations to compete with Cruz's vast national donor network.

“We’re not hiring pollsters, we’re taking no corporate cash, no PACs,” O'Rourke said.

“I don’t think Ted Cruz is a bad person, but we all know he spent four years running for president,” O’Rourke added. “And he did a really good job at it. That’s good for him, it’s just not great for Texas.”

This story has been updated.