Story highlights Cruz is the junior senator from Texas

Cornyn declined to endorse him in his GOP primary

Washington (CNN) Texas Sen. Ted Cruz shouldn't expect much help from the senior GOP senator from his state, John Cornyn, ahead of a potential Senate primary challenge next cycle.

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.

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