Republicans who want to participate in the first presidential debate of the 2012 primary cycle will be forced to formally get into the race by the end of this month, Fox News is expected to announce this afternoon.

Fox News Channel, a co-sponsor of May 5's debate with the South Carolina GOP, is expected to outline criteria this afternoon that would, most importantly, force candidates to make a decision by the end of the month about whether to get in the race if they want to participate in the debate.

Fox is expected to outline six criteria for candidates, which are listed below. The candidates and prospective participants in the debate:

— Must register a presidential exploratory committee or have announced a formal campaign for president;
— Must file all necessary paperwork with the Federal Elections Commission (FEC);
— Must file all necessary paperwork with the South Carolina Republican Party;
— Must have paid all federal and South Carolina filing fees;
— Must meet all U.S. Constitutional requirements; and
— Must have garnered at least an average of 1 percent in five national polls based on most recent polling leading up to April 29.

The criteria could mean a smaller crowd onstage than is so far expected for the debate, which will be held in Greenville, S.C.

Former Minnesota Gov. Tim Pawlenty, former House Speaker Newt Gingrich (Ga.), Rep. Ron Paul (Texas), former Sen. Rick Santorum (Pa.) and former Louisiana Gov. Buddy Roemer have all committed to participating in the debate, but only Pawlenty and Roemer have filed with the FEC to establish an exploratory committee; others are flirting with running, and have declined to formally enter the race. 

Other candidates could enter the race in the meanwhile; figures like Indiana Gov. Mitch Daniels (R) and Mississippi Gov. Haley Barbour (R), for instance, have said they'll decide on running at the culmination of their state legislatures' sessions, which would give them time to jump into the race before the April 29 deadline.

The South Carolina debate is set to be the first of the long 2012 campaign. A previous debate set for May 2 at the Ronald Reagan Presidential Library was postponed until the fall for lack of candidates.