The Trump-backed Tony Gonzales was trailing Cruz's pick, Raul Reyes, by just 11 votes out of 24,533 with all polling locations reporting, according to unofficial results. Gonzales' campaign said the race was not over.
"We will fight all the way to the end to ensure the integrity of this election and that every legal vote is counted," Gonzales campaign spokesperson Matt Mackowiak said in a statement.
Hurd's district is one of Democrats' best pick-up opportunities this fall nationwide. The party's nominee is Gina Ortiz Jones, who lost to Hurd by a razor-thin margin in 2018.
Gonzales, a former Navy cryptologist backed by Hurd and House leaders, nabbed Trump's endorsement earlier this month, just three days after Cruz endorsed Reyes. Cruz had lobbied Trump to stay out of the runoff.
Trump also starred in a robocall for Gonzales that went out Monday and held a tele-town hall with him on the eve of the runoff.
Trump had endorsed another candidate in the Texas runoffs, Ronny Jackson, the former White House doctor running to replace retiring U.S. Rep. Mac Thornberry, R-Clarendon. Jackson easily defeated his rival, Josh Winegarner, on Tuesday night.