Jackson is running in the state's 13th congressional district, a Republican stronghold and seat that will be left vacant by Rep. Mac Thornberry (R ), the ranking member of the House Armed Services Committee, who announced his retirement last year.
Jackson entered the crowded primary late, turning in paperwork a day before the deadline to file.
"I hope we can get Admiral @RonnyJackson4TX of Texas, who served our Country so well, into the runoff election in #TX13!" Trump tweeted. "Ronny is strong on Crime and Borders, GREAT for our Military and Vets, and will protect your #2A. Get out and vote for Ronny on Tuesday, March 3rd!"
The Texas primary takes place on Super Tuesday.