Mace was the first female graduate of the Citadel military university, and went on to co-found a libertarian-leaning news site called FITSNews. She's a longtime Ron Paul supporter and has ties to the Tea Party in South Carolina.
She joins businessman Richard Cash in the primary, and others still, including state Sen. Lee Bright (R), are looking at the race as well.
Conservatives have long named Graham as a target for a primary challenge due to his support of immigration reform and the financial bailout, among other votes. He's also criticized the no-new-taxes pledge from Americans for Tax Reform, drawing a rebuke from ATR President Growver Norquist.
But Graham has worked to defend himself against a potential primary challenge. He has more than $6.3 million in the bank, and polls have shown him relatively well-poised to retain his seat next year.