Donald J. Trump made his first congressional endorsement this weekend, backing Representative Renee Ellmers in a North Carolina primary race in which she has been opposed by heavily funded adversaries.

Mr. Trump recorded a phone message for Ms. Ellmers’s campaign, calling her a “fighter.”

“I need her help in Washington so we can work together to defeat ISIS, secure our border, and bring back jobs and frankly, so many other things,” Mr. Trump said, referring to the Islamic State. “And Renee knows how to do it. She gets it. And together, we will make America great again.”

Ms. Ellmers is running in a redrawn congressional district against another incumbent, Representative George Holding, and a third candidate, Greg Brannon.

Ms. Ellmers has been a target of conservatives for many months, and she has been opposed by Americans for Prosperity, the group affiliated with David H. and Charles G. Koch, and by the anti-tax Club for Growth.