According to a newly-announced lineup by debate host Fox Business Network, only seven candidates will appear in the main debate: Donald Trump, Ted Cruz, Marco Rubio, Ben Carson, Jeb Bush, Chris Christie and John Kasich.
Rand Paul's campaign tells ABC News that he will not participate in the debate. "By any reasonable criteria, Senator Paul has a top tier campaign," his campaign said in a statement Monday night. "He will not let the media decide the tiers of this race and will instead take his message directly to the voters of New Hampshire and Iowa."
Fox Business says Paul's campaign has declined their invitation. Mike Huckabee and Rick Santorum will join Fiorina in the undercard debate.
Fox Business said they would include the candidates who are either in the Top 6 nationally, or the Top 5 in either Iowa or New Hampshire. The network used a total of 17 different polls in determining the three different ways candidates could make the mainstage debate.
Paul trailed Bush for the final spot in the debate nationally and in Iowa by 0.8 and 0.6 percentage points, respectively, according to an ABC News analysis.
"If you tell a campaign, with three weeks to go they are second tier, you destroy the campaign," Paul told the "Kilmeade & Friends" radio show on Dec. 23.
Fiorina, who appeared in the first undercard debate hosted by Fox News in August, delivered a strong performance and was added to the mainstage debate. After a bump in polling, especially in New Hampshire, the former Hewlett-Packard CEO has since lost substantial support.
Debate format has long been a contested topic during the 2016 presidential cycle, as some candidates have pointed to small differences in polling averages as an unfair standard.
Full averages, according to an ABC News analysis, are below:
National: Trump: 35.0 Cruz: 19.6 Rubio: 11.0 Carson: 9.4 Christie: 4.0 Bush: 3.6 Paul: 2.8 Fiorina: 2.2 Kasich: 1.8 Huckabee: 1.2 Santorum: 0.5 Gilmore: 0.3
Iowa: Cruz: 27.8 percent Trump: 26.4 Rubio: 14.0 Carson: 9.4 Bush: 4.8 Paul: 4.2 Christie: 3.2 Fiorina: 2.0 Huckabee: 1.8 Kasich: 1.4 Santorum: 0.6 Gilmore: 0.1
New Hampshire: Trump: 29.6 percent Rubio: 12.8 Cruz: 11.6 Christie: 9.6 Kasich: 9.0 Bush: 8.0 Carson: 4.4 Fiorina: 4.0 Paul: 3.8 Huckabee: 0.6 Santorum: 0.2 Gilmore: 0.2
ABC's Jessica Hopper contributed to this report.