And on Wednesday, those months came to a head: Castro and O'Rourke fought in arguably the most heated exchange of an otherwise mild Democratic debate over immigration, an issue both candidates view as central to their campaigns.
Underneath the back-and-forth was the personal relationship between the two Texans. Although they have worked together in the past -- and Castro campaigned for O'Rourke when he ran for Senate in 2018 -- it appeared to many in the state that the El Paso congressman had eclipsed Castro as the state's pre-eminent Democrat during that run against Republican Sen. Ted Cruz.
Castro famously keynoted the 2012 Democratic National Convention, earning plaudits that cast him as Texas' rising star and possibly the next President Barack Obama. But O'Rourke, with an upstart Senate run, became a national Democratic icon in 2018, taking that mantle from Castro and earning the support of many of the same people that launched Obama.
"They're friendly but clearly competitive," said a Castro aide after their back-and-forth on stage. Asked what happened to a once closer relationship, the aide added: "Ambition."
Castro opened the immigration section of the debate by highlighting that he released an immigration plan in April -- the first of the 2020 candidates -- that centered on repealing Section 1325 of the Immigration and Nationality Act, which was used by the George W. Bush administration to apply a criminal violation -- as opposed to a civil infraction -- to anyone entering the United States illegally.
The decades-old law was used by the Bush administration to attempt to curb the number of illegal entrances into the United States, something that Castro said needs to be repealed.
"Let's be very clear, the reason that they are separating these little children from their families is that they are using section 1325 of that act, which criminalized crossing over the border, to incarcerate the parents and then separate them," Castro said.
"Some of us on this stage have called to end that section, to terminate it, some, like Congressman O'Rourke, have not," Castro said, turning and gesturing towards the congressman.
With both candidates on the screen, O'Rourke quickly moved to respond but Castro talked over him.
"And I want to challenge all of the candidates to do that. I just think it is a mistake, Beto, and I think if you truly want to change the system then we have to repeal that section. "
O'Rourke, turning to Castro, got in his response: "Actually, as a member of Congress, I helped to introduce legislation that would ensure we don't criminalize those who are seeking asylum and refuge in this country."
Castro was not talking about asylum seekers, however, a point he made clear to the former congressman.
"You are looking at just one small part of this," O'Rourke said. "I am talking about a comprehensive re-write of our immigration laws."
"That's not true. That is actually not true," Castro said, looking at O'Rourke and adding that the congressman recently told CNN that he did not think the law should be repealed because of the way he believes it would hurt the fight against drug and human trafficking.
But what Castro noted is that there are other codes in the same law that focus on trafficking.
And then, as the conversation came to an end and as others talked over him, Castro delivered his hardest blow at O'Rourke: "I think you should do your homework on this issue, if you did your homework on this issue you would know we should repeal this section."
Both candidates in split screen for most of the exchange and the moderators left them continually go at it.
But Castro was not done.
In an interview with CNN after the debate, Castro made his point clear: "The thing about Congressman O'Rourke is that he clearly hasn't done his homework."
"He is claiming a couple of days ago that we need to keep that law in place because we need to protect the country from human traffickers or drug traffickers," Castro said. "I pointed out that we have other laws that specifically relate to people who engage in drug trafficking or human trafficking. And the only way that we're going to end family separations is if we get rid of that law."
He later told CNN's Anderson Cooper that O'Rourke "failed" on immigration policy.
O'Rourke responded to Castro in the spin room after the debate.
"I think we have the same goal. I think he's got it wrong on how he describes my position. I'm focused on the big picture: making sure our laws reflect our values, our interests, and what we know to be right in this country of immigrants, asylum-seekers and refugees," O'Rourke said.
And on Castro saying that he hasn't done he homework, O'Rourke responded: "Well, he's wrong on this one."
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