"Dan Branch is a long-time friend of Texas' Hispanic community," Antuna said in a statement. "As a top-rated attorney and accomplished Republican legislator, Dan Branch is far and away the most qualified candidate for attorney general."
Branch is facing state Sen. Ken Paxton, R-McKinney, in a runoff that will be held on May 27.
In the March election, Branch earned about 33.5 percent of the vote — compared with Paxton's 44.4 percent.
Barry Smitherman, the chairman of the Texas Railroad Commission, also competed in the three-way primary race, but with only 22.1 percent of the vote in March, did not make it into the runoff.