Shannon is running in the primary against Rep. Jim Lankford (R-OK).
The endorsement is Cruz's first of the cycle. Cruz has vowed to stay out of primaries where tea party candidates are challenging incumbent Republicans but since Shannon, a favorite of some of the outside groups like the Senate Conservatives Fund (SCF), isn't challenging an incumbent senator, Cruz isn't violating that promise.
Cruz is a vice chairman of the National Republican Senatorial Committee and that position has sometimes created a tension with his association to SCF given that SCF is trying to boost candidates that the NRSC wants to be reelected. The NRSC is staying neutral in the Oklahoma Senate race.
According to Republican campaign officials, the NRSC has met with Shannon a few times and have come away feeling "impressed" with him.
Shannon was also endorsed by former vice presidential candidate Sarah Palin.
The NRSC is officially neutral in the race and Cruz did not endorse Shannon in his capacity as vice chairman.