A spokeswoman for Senator Cruz says he had already made it abundantly clear that he would not get involved in any incumbent primary elections.
University of Houston Associate Professor of Political Science Brandon Rottinghaus says this is a form of polite payback for Sen. Cornyn not taking sides in last year's Republican primary for Senate.
"Going back to the election where Ted Cruz was sort of a surprise victory over (Lt. Gov.) David Dewhurst, and Cornyn sat out much of that, I think there is some friction, if not outright hostility about this."
But Cruz's people point out that he has already donated $2,500 to Cornyn's re-election campaign. And Professor Rottinghaus says the two do have a fairly good working relationship — agreeing on most issues, if not tactics.
"Sen.Cruz's tactics have been very different than Sen. Cornyn's. So, one could consider Sen. Cornyn sort of a more establishment Republican, willing to work within the system and work with the leadership where we've seen Sen. Cruz is less likely to do that."
Sen. Cornyn is strongly favored to win the nomination. Rottinghaus says when that happens, Sen. Cruz will come out with what Rottinghaus calls a "non-endorsement endorsement" — in which he'll favor Cornyn because he's a Republican, but he won't be running ads or raising money for him.