Editor's note: This story has been updated throughout.
Castro, a San Antonio Democrat, is mulling a challenge to U.S. Sen. Ted Cruz's re-election bid, along with U.S. Rep. Beto O'Rourke, D-El Paso. Should Castro pass on a run, Vela told the Tribune in a Monday morning interview in Austin that he would back O'Rourke.
"I will support Joaquin Castro for whatever he runs for. If Joaquin does not run, I will fully support Beto O'Rourke," Vela said in the interview with Tribune CEO Evan Smith. "In the meantime, I am introducing both to the constituencies that I represent because I think that it's only fair that as Democrats prepare for the 2018 cycle that they hear from all of the candidates."
There is no certainty that there will be a competitive Democratic primary — neither man has formally declared a Senate campaign. And there's an assumption within Texas Democratic circles that O'Rourke is more likely to follow through on a run. Castro, O'Rourke, Vela, who are all in the same congressional class, are in their third terms.
Castro said over the weekend he would decide on a candidacy within eight weeks.
However, the fact that a member of Congress is willing to lend his support to Castro is indicative that a Senate Democratic primary is not out of the question.
The Texas Senate race is slowly picking up national interest, and it is one of the few places where Democrats are on offense in 2018. But even with the interest of two of the state's fastest-rising Democratic stars, a race against Cruz begins as an uphill climb for the party. Vela said as much on Monday, conceding it will be "a tough road" no matter which Democrat runs.
There is little hope within the state that the national party is prepared to dump the enormous amount of money needed to compete for the seat.
"We have to understand that even the 2018 cycle, given the past gubernatorial cycles, it's a tough road for whoever runs," Vela said.
During Monday's interview, Vela also laid out a strategy that other state Democrats are privately discussing: the need to compete beyond the state's urban strongholds.
A common argument taking hold in those circles is that while no Democrat sees a scenario of dominating the rural and suburban regions, the party must increase turnout in those places to boost statewide margins.
"If we're going to win statewide, we've got to begin to ... demonstrate a stronger showing in rural Texas as well," Vela said.
Vela is a key player in Texas Democratic politics. Increasingly, he is becoming a power broker in South Texas. But also, he is a prolific fundraiser with deep ties to his region's donor community.
Beyond politics, Vela reiterated his harsh criticism of President Donald Trump and his advocacy for a border wall.
"It doesn't make any sense to me why you would ever build a wall," Vela said. "In fact, I would tear the existing wall down, but I'm realistic, too. ... I know that's not likely going to happen, and to the extent that the administration wants to build strategic fencing or further walls, I'm going to resist that as much as I can."
Vela pointed to the Senate as the pressure point for blocking expansion of the wall.
Asked if Trump is a racist — a statement he made in June with an infamous open letter to then-candidate Trump — Vela reinforced that charge: "I think when you consider all the comments he's made, when you take a look at these enforcement priorities, I don't know how you can come to any other conclusion."
In that June missive, Vela suggested that Trump shove the border wall up his "ass." Pressed on the nature of that language, Vela was unrepentant Monday.
"I am a nice guy, but I don't think I could have said it another way, and I meant it, and I mean it today because I feel so strongly about not just the border wall but about the U.S.-Mexico relationship," he said.
"My view is that the wall is a horrible symbol when we begin to talk about the U.S.-Mexico relationship, and for that reason, I'm adamantly opposed to it."