Fox Business Network has tightened the criteria for participation in the primetime portion of its Jan. 14 debate among Republican presidential hopefuls. Under the new rules, the primetime debate is likely to be limited to a field of six or less.

The rules stipulate that the primetime debate contenders have meet at least one of three eligibility requirements: place in the top six in an average of the five most recent national polls released by Jan. 11; place in the top five in Iowa in an average of five recent polls; or place in the top five in New Hampshire in an average of five polls.

As the poll rankings stand today, Fox Business estimates that six candidates would qualify for the primetime portion. The Dec. 15 GOP debate on CNN featured nine participants in the primetime telecast.

Fox Business will also host an undercard debate earlier in the day on Jan. 14. Eligibility in the earlier debate will be set at candidates achieving at least a 1% showing in the five most recent national polls.

The Jan. 14 debate at the North Charleston Coliseum in Charleston, S.C., will focus on economic, domestic and international policy issues. Fox Business first GOP debate, held Nov. 10, set a ratings record for the cabler with 13.5 million viewers.

The field of Republican contenders for the presidential nomination is already thinning out as the Iowa caucuses draw near. On Monday, Sen. Lindsey Graham of South Carolina ended his bid for the White House.