David Jackson

USA TODAY

The Fox Business Network announced on Monday it was inviting only seven Republican candidates to its prime-time debate later this week, relegating Rand Paul and Carly Fiorina to a preliminary session.

The main debate in North Charleston, S.C., on Thursday night will feature businessman Donald Trump, Texas Sen. Ted Cruz, Florida Sen. Marco Rubio, retired neurosurgeon Ben Carson, former Florida governor Jeb Bush, New Jersey Gov. Chris Christie and Ohio Gov. John Kasich.

Paul, a Kentucky senator who has been on the main stage in five previous debates, told CNN he will likely skip the 6 p.m. ET preliminary debate Thursday: "I won't participate in anything that's not first tier because we have a first-tier campaign."

Paul and Fiorina were invited to the "undercard" debate along with former Arkansas governor Mike Huckabee and former Pennsylvania senator Rick Santorum.

Earlier in the day, the Paul campaign said in a statement that he placed fifth in a recent Iowa poll and has finished ahead of Bush, Christie, Kasich and Fiorina in other surveys.

Citing criteria announced last month, Fox Business Network said that, "to qualify, a candidate needs to be either among the top six in an average of the five most recent national polls, or among the top five in an average of the five most recent Iowa or New Hampshire polls."

The network ended up using six polls for the national average and the Iowa average because the two earliest polls were done concurrently. They also noted that the polls used were from non-partisan, nationally-recognized organizations using standard methodological techniques.

"We only used the most recent results from a particular survey organization so that no single pollster dominates the calculation of averages," Fox said in a memo on the debate selection.

Fiorina, a former Hewlett-Packard CEO, participated in the undercard forum at the first GOP debate of the 2016 campaign in August. After a widely praised performance, she vaulted to the main stage for subsequent debates. However, her poll numbers have steadily declined since the fall, and recent national surveys as well as those in early voting states have put her in low single digits.

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