Georgia Democratic U.S. Senate nominee Michelle Nunn is out with a new TV ad in which she directly rebuts her Republican rival David Perdue’s claim that she “funded organizations linked to terrorists” while running former President George H.W. Bush’s Points of Light Foundation.

“That’s a terrible lie and an insult to the millions of volunteers I worked with to make a difference,” Nunn says in the new ad. “David Perdue’s ad has been called the worst in America and President Bush’s son called it ‘shameful.’”

Perdue’s ad featured footage of ISIS fighters and claimed that “Michelle Nunn’s own plan” tied her to terrorism-linked funding.

Fact checkers have not been kind to that commercial, which refers to a leaked memo from Nunn’s campaign that warned Republicans would try to link grants from Point of Light to an Islamic charity to terrorism. But the memo merely predicted the attack, it didn’t say it was accurate and nonpartisan outlets that have looked into it have slammed Perdue’s claims as blatantly misleading. Politifact gave his ad a “Pants on Fire” rating while the Washington Post’s Fact Checker blog gave it “Four Pinnochios,” with each concluding that the claim Nunn’s charity gave money to a terrorist-linked charity was wildly off base.

A number of Republicans have emphasized national security in recent weeks as military operations against ISIS have escalated. In New Hampshire, Republican Senate candidate Scott Brown has run ads that, like Perdue’s, warn ISIS could infiltrate the Mexican border. Homeland Security Secretary Jeh Johnson has said there is “no specific, credible threat” emanating from the border.

Recent polls show Perdue holding onto a small lead after emerging from one of the toughest Republican primary fields of the election in July.