SAN ANTONIO — After U.S. Rep. Joaquin Castro wrapped up speaking here Wednesday, completing the latest stop on his statewide tour ahead of a potential Senate run, one man in the crowd turned to another and voiced some ambivalence.

"I don't know if he should do it," the man said, alluding to what would be an uphill battle against U.S. Sen. Ted Cruz, R-Texas. "He's got a good thing going."

It's the question hanging over the San Antonio Democrat as he nears an announcement on his plans for 2018: Is it worth giving up his seat in Congress, where he has had a steadily growing profile, for a long-shot challenge of Cruz, particularly when another Democrat, U.S. Rep. Beto O'Rourke of El Paso, is already running?

"At the end of the day, it's all about what's in his gut," said Julián Castro, Joaquin Castro's twin brother and the former U.S. housing secretary. "Is this something where he can best serve the people of Texas and his constituents, and that's not an easy decision because he's done a lot of great work in Congress and he has significant committee assignments that allow him to serve his constituents and the American people well."

Joaquin Castro moved from the Texas House to the U.S. House in 2013 and in a short time gained coveted spots on the Foreign Affairs Committee and the Intelligence Committee, a panel that has put him on the front lines of congressional efforts to investigate Russian interference in the 2016 election. He also sits in a safe Democratic district where he could easily win re-election in 2018 and beyond — O'Rourke, by contrast, has term-limited himself.

Castro has maintained a somewhat elastic timeline for announcing whether he plans to run. He first said Feb. 11 that he would reveal his intention in eight weeks, a deadline that passed without any announcement. He then began saying he would make his plan known by the end of April. Rallying supporters Wednesday in San Antonio, he opened the door to a longer timeline, saying he will "take a few more weeks to come to a decision and make an announcement."

"I always said around the end of April," Castro said after the rally as reporters peppered him with questions about the timeline. "I set out a timeline for myself a long time ago, and I've stuck with that timeline."

Cruz's team, meanwhile, has been happy to taunt Castro throughout his decision-making process. Cruz himself suggested in February that Castro would be "retired from public service" if he sought the Senate promotion.

"Like sand through an hourglass, so are the numbered days before Mr. Castro publicly admits that he will never run for anything more than a safe seat in Congress," said Jeff Roe, a senior adviser to Cruz. "He was out-maneuvered by the mighty Irishman O'Rourke."

Yet Julián Castro said Wednesday his brother remained genuinely undecided on whether to challenge Cruz. "For sure," he said after the stop in San Antonio.

O'Rourke, for his part, has plowed ahead full-steam with his campaign as Castro continues mulling a run. Since announcing his bid on March 31, O'Rourke has held campaign events in 12 cities across the state. He is scheduled to hit another seven cities through Monday. Castro has made public appearances in at least five Texas cities during the same period, including four outside of his congressional district.

Asked about Castro's potential candidacy at events, O'Rourke has largely expressed deference, saying that the two have long shared their interest in the race with one another and that if Castro runs, they will compete in a way that "makes Texas proud." O'Rourke said Friday he has no problem if Castro wants to take longer than his original timeline to make up his mind, saying he wants the San Antonio congressman "to do what's right for him, for his family and what he thinks is best for the country."

At the same time, however, O'Rourke has shown awareness that a strong start to his campaign could have an impact on the trajectory of the primary.

"If you don't want anybody else to run and you want to make sure we're concentrating all our resources, all of our focus, all of our dollars, on seizing a historic, once-in-a-30-year opportunity," O'Rourke said this month during a campaign stop in San Marcos, "then get behind me."

The encouragement is "less about anyone else than our effort," O'Rourke said Friday.

Whoever runs on the Democratic side, national Republicans continue to express confidence that the seat will easily hold for the party in 2018. Democrats have mostly put Texas on the back burner, as the party remains concerned about 10 Senate incumbents who represent states Donald Trump carried last year.

Senate races are frequently so highly organized that they can often resemble the sophistication of a presidential campaign. It is difficult to quietly plan a Senate campaign — and the chatter around Castro in both House Democratic and Senate circles is remarkably quiet in Washington.

In the donor community, there have not been many outward signals that the moneyed class is rallying around either candidate for the time being, potentially waiting to see what Castro does. The San Antonio congressman spent the past several years traveling across the country for the Clinton campaign, and he frequently fundraises for his colleagues — giving him a well-heeled network that would probably kick into higher gear were he to run for Senate.

Amber Mostyn, a Houston megadonor who was a key political patron of 2014 gubernatorial candidate Wendy Davis, said this month she had no preference between Castro and O'Rourke. She added that she was not worried about a contested primary, saying Texas Democrats "just need to beat Ted Cruz."

"I love a good, healthy primary, so I look forward to seeing whoever’s in the race," Mostyn told reporters in Houston. "As long as they’re talking about good, progressive issues that help working families in Texas, that help women in Texas, I’ll be there to support them."

O'Rourke has at least one big name in his corner: Marc Stanley, a Dallas attorney and one of Texas' top Democratic fundraisers. Stanley gave $2,700 — the maximum amount under federal law — to O'Rourke the day he launched his Senate campaign. Stanley said Friday he is "going to support Beto for the primary" but would back Castro "if he ends up being the nominee."

Castro and O'Rourke raised similar amounts — $219,000 and $208,000, respectively — during the first quarter of the year, according to recently released campaign finance reports that covered only the first day of O'Rourke's Senate campaign, March 31. Castro's report also featured some boldfaced names, including Arthur Schechter, a Houston attorney and one of the top Democratic donors in Texas.

"As you can imagine, Joaquin has received strong encouragement and support from the grassroots and donors in every part of Texas," Castro political director Matthew Jones said in a statement. "We're thankful for it, and an announcement of those specific supporters would come at the same time as an announcement of Joaquin's candidacy for the United States Senate."

Abby Livingston contributed to this report.

Disclosure: Amber Mostyn has been a financial supporter of The Texas Tribune. A complete list of Tribune donors and sponsors can be viewed here.

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