Pending redistricting shifts, the decision will make his district a GOP target in the midterm elections.
He's leaving a safe seat in an attempt to unseat incumbent Republican Ken Paxton
No candidate received a majority of the vote on November 2; this runoff will fill a vacancy
He has been the state's at-large House representative since being elected in 2006.
Now in her 15th term, she represents a safely Democratic Dallas-area district