Nancy Mace, one of the first women to graduate from The Citadel, will mount a Republican primary challenge against Sen. Lindsey Graham in South Carolina next year, marking the most serious threat so far to Graham’s re-election.

Mace will make her official announcement Saturday at a breakfast event in Goose Creek, S.C., she suggested in a post on her website.

Graham has spent considerable time and resources to defend against such threats, most notably by building a massive campaign fundraising operation that ended the most recent quarter with more than $6.3 million cash on hand.

But Mace will challenge Graham from his right flank, raising questions about the deals he has cut with Senate Democrats on issues like comprehensive immigration reform. She joins Richard Cash, a Republican businessman, who has already announced his candidacy. A third Republican, state Sen. Lee Bright, is also weighing a bid.

Already, Mace has “been working the D.C. crowd for months,” one South Carolina source said, making phone calls to potential supporters, holding meetings — including one with the Senate Conservatives Fund, the super PAC formerly led by Jim DeMint — and generally boosting her name recognition.

She has met also with the influential Club for Growth, a source within the organization confirmed, although the source would not offer further details about when the meetings took place.

“She has star potential,” the South Carolina source said. “She can be the Rubio, Cruz of this cycle,” referring to Sens. Marco Rubio, of Florida, and Ted Cruz, of Texas, who won tough Senate contests — and, later, national notoriety — with backing from conservative outside groups and Tea Party supporters.