Four GOP candidates sign up for DeMint presidential forum

UPDATE: Former House Speaker Newt Gingrich also accepted an invitation to participate in the forum.

(CNN) - Four presidential candidates have accepted invitations to participate in a Labor Day forum in South Carolina being organized by Sen. Jim DeMint, a conservative kingmaker who has yet to pick a horse in the 2012 race.

Texas Gov. Rick Perry, Minnesota Rep. Michele Bachmann, Texas Rep. Ron Paul and businessman Herman Cain have agreed to take part in the Palmetto Freedom Forum, to be held on Sept. 5 at a convention center in downtown Columbia.

Invitations to the event went out to eight candidates Monday, a Republican source told CNN.

Also invited were former Massachusetts Gov. Mitt Romney and former House Speaker Newt Gingrich, along with two Republicans still flirting with a bid: former Alaska Gov. Sarah Palin and former New York City Mayor Rudy Giuliani.

But event organizers - a small group of well-connected DeMint supporters calling themselves the "Keep Your Powder Dry Caucus" as they search for a 2012 candidate to get behind - did not invite to two declared candidates, former Pennsylvania Sen. Rick Santorum and former Utah Gov. Jon Huntsman.

Organizers said early on that to be invited, a candidate must surpass the 5 percent threshold in the RealClearPolitics average of national polls. Santorum and Huntsman have yet to do so.

The forum, officially titled the "American Principles Project Palmetto Freedom Forum," will be fronted by DeMint, Iowa Rep. Steve King and Robert George, a conservative legal scholar and the founder of the American Principles Project.

Candidates will take turns fielding questions from the trio of panelists for roughly 20 minutes at a time.

According to organizers, candidates will be asked to "engage in a thoughtful, substantive discussion of their stances on critical issues facing our country."