Rep. Scott DesJarlais, R-Tenn., was declared the winner of his GOP primary Thursday night.

The three-term congresswoman beat 28-year-old Grant Starrett, a former aide to Mitt Romney. DesJarlais secured 51 percent of the vote to Starrett's 44 percent when the Associated Press called the race around 10:10 p.m. local time.

Starrett had been endorsed by National Review, a conservative publication. He put nearly $1 million of his own money into the campaign, but was unsuccessful in unseating the incumbent, who painted the challenger as a "trust fund millionaire."