Editor's note: This story has been updated.
Castro, long regarded as a rising star in Democratic politics, has previously not ruled out a run for the seat but seemed to offer more definitive language than usual in a TV interview Tuesday morning.
"I’m going to take a look at it in 2018," Castro told CBS News, which interviewed him here at the Democratic National Convention with his twin brother, U.S. Housing Secretary Julián Castro.
"I’ll take a look at that and other opportunities," Joaquin Castro added. "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."
By Thursday afternoon, Cruz was using Castro's comment to raise money for his re-election campaign, writing on Twitter that the "time to fight back is now to stop any attempt to turn Texas blue."
"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 said in a subsequent email to supporters, going on to note that "our opponents are already making plans to defeat us, so we cannot afford to wait to engage until 2018."
Cruz's campaign further pressed its case in a statement later Tuesday, calling the senator 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 the candidates in Texas who embrace the same corrupt, big-government, anti-liberty policies of their nominee Hillary Clinton," Cruz spokeswoman Catherine Frazier said. "We'll gladly contrast that dismal vision with the record and principles Sen. Cruz has promoted from day one — 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."
In the interview, Joaquin Castro appeared more interested than his brother in challenging Cruz. Asked by CBS' Charlie Rose which one of the twins was going to take on Cruz, Julián Castro replied, "Probably zero of us."
"He's speaking for himself," Joaquin Castro said a short time later.