Smitherman, the chairman of the Texas Railroad Commission, came in third in the three-way Republican primary in March. Branch and state Sen. Ken Paxton, R-McKinney, are currently in a runoff.
"I did not come to this endorsement lightly," Smitherman wrote. "My decision to run for Attorney General was based upon our family’s belief that I brought skills, experience, and values that the other two candidates did not possess to the same degree."
He indicated that his decision to issue an endorsement was prompted in part by a recent reprimand and fine Paxton received from the State Securities Board for violating the Texas Securities Act. Smitherman called the development "very troubling."
"While experience, leadership, and legal skills are important, perhaps the most important of all qualities needed to be Attorney General, the chief law enforcement officer in the State of Texas, is an unwavering commitment to compliance with both the letter and the spirit of the law," he wrote. " The Attorney General must have integrity, honesty, and high moral character."
Smitherman noted that there are unanswered questions related to Paxton's past business practices. Paxton has not commented on the matter in detail since it was first reported in the Tribune in April.
Smitherman concluded that he was endorsing Branch "for these and other reasons."
Whoever emerges victorious in the Republican runoff on May 27 will face Democrat Sam Houston in the general election.