Updated at 9:33 a.m. Wednesday with response from a Cruz spokeswoman.

WASHINGTON — Rep. Joaquin Castro said Tuesday that he is considering running for the Senate against Sen. Ted Cruz in 2018.

Castro, a San Antonio Democrat, was on CBS This Morning with his brother, Secretary of Housing and Urban Development Julián Castro, when host Charlie Rose asked if either planned to challenge Cruz.

At first, Julián Castro denied any collective interest. Although Cruz's "standing has fallen fairly far," he said, as of now, "that's not in our plans to run in Texas." But then Joaquin Castro interjected.

"He's speaking for himself," Joaquin Castro said. "I'm going to take a look at it in 2018, I'll take a look at that and other opportunities. I've never been somebody that said, 'In two years I've absolutely got to run for Senate or governor,' but I will take a look at it."

Castro has not outright dismissed the possibility of a Senate run before, but his comments Tuesday were his clearest indication yet that he is actively exploring the option.

The Castro brothers have long been considered the brightest prospects for Texas Democrats to win back statewide office for the first time since 1994. That challenge remains a steep uphill climb, as demonstrated by Republicans’ comprehensive victories in 2014, including a 20-point win for Greg Abbott over Wendy Davis in the gubernatorial race.

Shortly after Castro made the comments, Cruz used the news in a fundraising appeal for his own Senate campaign.

"Despite any record of achievement and a liberal ideology out of touch with the majority of Texans, the Castro brothers have the full support of the mainstream media and Washington establishment willing to do everything in their power to turn Texas blue," Cruz wrote in an email to supporters. "Our opponents are already making plans to defeat us, so we cannot afford to wait to engage until 2018."

Cruz spokeswoman Catherine Frazier added in a statement late Tuesday that the senator remains "the champion Texas needs to continue battling the Washington corruption empowered by the Obama-Clinton machine."

"No doubt the Democrats will attempt to prop up candidates in Texas who embrace the same corrupt, big-government, anti-liberty policies of their nominee Hillary Clinton," Frazier said. "We'll gladly contrast that dismal vision with the record and principles Sen. Cruz has promoted from Day 1 based on defending the Constitution and fighting for jobs, freedom and security - the very policies that have enabled Texas to thrive in spite of an ever-encroaching federal government."

Julián Castro also dismissed another potential future job he had been suggested for in recent days: chairman of the Democratic National Committee.

“I have no interest in that,” Julián Castro said. But this time he left room for his brother to give his own response. “Since I didn’t speak well for him the first time, he might want to speak for himself the second time,” he said.

"Yeah, you never say never," Joaquin Castro responded. "But it hasn't been on my radar."

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