Cornyn, the Senate majority whip, told CNN Wednesday that he would not endorse either Cruz or his possible primary challenger, Rep. Michael McCaul, if the two squared off in 2018. It's unusual for senators not to endorse colleagues from the same party -- let alone from the same state.
But Cornyn's decision to stay out of a primary challenge shows how frosty Cruz's relations have become with his GOP colleagues after three years of bitter infighting and name-calling from the campaign trail. Cruz himself declined to endorse Cornyn during the senior senator's 2014 primary campaign.
"I'm not going to get involved in any primary races particularly with my colleagues in the Senate," Cornyn told CNN when asked about Cruz and McCaul squaring off. "We'll see what happens, but we've got plenty to worry about between now and November 2016 before we start worrying about November 2018."
Reminded that senators typically back incumbents from their own party, Cornyn demurred.
"I see no benefit to anyone to get involved in the contested Republican primary so I would sit on the sidelines and watch with interest," Cornyn said, adding that he is "not recruiting anybody" to get into the race.
Cornyn's comments come as a behind-the-scenes lobbying campaign to recruit McCaul into the race has begun to take shape, intensifying after Cruz's speech at the GOP convention in July when he refused to endorse GOP presidential nominee Donald Trump
-- eliciting backlash in some quarters of the GOP.
Yet defeating Cruz in a primary challenge would be a tall order, given the deep support he has from grassroots conservatives and the national fundraising network he built during his presidential campaign this year.
McCaul, however, is one of the wealthiest members of Congress. And he has positioned himself as a leading GOP voice on national security issues as chairman of the House Homeland Security Committee.
On Wednesday, McCaul declined to say if he would rule out a run against Cruz after November.
"I'm focusing on reelection," McCaul told CNN when asked if he was considering a run against Cruz. "That's my focus right now."
Cruz has largely avoided weighing in on McCaul, but he sent out a fundraising notice last week saying that the establishment is looking to fund a primary challenge against him.
On Wednesday, Cruz declined to comment about McCaul as he was walking into a hearing.
"I'm headed to a Judiciary Committee hearing," he said.