Appointed U.S. Sen. Darline Graham conceded she is "not that informed on national security" during a tense Tuesday night debate against Rep. Ralph Norman, the clearest stumble yet in a runoff campaign that has repeatedly tested a first-time candidate thrust into her late brother's old job.

Asked directly whether Taiwan and the South China Sea represent national security concerns for the United States, Graham told the Newberry audience, "I'm just going to be honest here — I'm not that informed on national security." She added that she was "not a polished politician up here" and that "national security is not my thing, not my area of expertise, but I do support the military."

"I'M NOT A POLISHED POLITICIAN UP HERE, NATIONAL SECURITY IS NOT MY THING, NOT MY AREA OF EXPERTISE, BUT I DO SUPPORT THE MILITARY."

The moment drew immediate scrutiny because of who her late brother was. Sen. Lindsey Graham, who died suddenly on July 11, spent decades as one of the Senate's most prominent foreign policy hawks on Ukraine, Israel and Iran. South Carolina Gov. Henry McMaster appointed Darline Graham to fill the seat weeks later, and President Trump both encouraged her run and endorsed her over Norman and eight other Republicans in the initial special primary.

A rocky night in Newberry

The national security exchange was not the only rough patch for Graham, according to NBC News' account of the debate. She misstated South Carolina's population as roughly 50 million residents — the state's actual population is about 5.6 million. Graham also leaned on her brother's record, noting he served 33 years in the Air Force, and pushed back against Norman by saying, "I've done more in 20 days than you have done in 20 years."

Norman, a five-term congressman from South Carolina's 5th District, cast himself as the anti-establishment alternative, telling the crowd "Washington is with her" in a bid to paint Graham as the insider despite her lack of prior office.

The Associated Press wire report carried by ABC News noted that Graham's answer exposed what has become the central vulnerability of her campaign: a lack of governing and policy experience just weeks into a Senate term she never sought before her brother's death.

Graham and Norman, who finished first and second respectively in the multi-candidate special primary, face off in a Republican runoff scheduled for next week. The winner will carry the GOP nomination into the November 6 general election against Democratic nominee Dr. Annie Andrews for the remainder of the elder Graham's term.