Republican candidates Rand Paul and Carly Fiorina didn't make the cut to participate on the main stage of the next GOP primary debate, which will be hosted by Fox Business on Thursday.

Fox Business announced Monday that the primetime Jan. 14 debate will include only the top seven candidates in the crowded Republican field. Those candidates -- Donald Trump, Ted Cruz, Marco Rubio, Ben Carson, Chris Christie, Jeb Bush and John Kasich -- will be featured at 9 p.m. EST. Meanwhile, Paul and Fiorina have been designated to the undercard debate at 6 p.m. EST.

According to criteria established by the network, neither the Kentucky senator nor the former Hewlett-Packard executive qualify to debate on the main stage based their average in the five most recent national polls. They also failed to get enough traction to be listed in the top five in Iowa or New Hampshire. As a result, they were relegated to the preliminary round, along with former Arkansas Gov. Mike Huckabee and former Pennsylvania Sen. Rick Santorum.

The announcement comes as a major blow for Paul and Fiorina since the first early states caucuses in Iowa are less than a month away.

In response, Paul said he will boycott the Republican primary debate in South Carolina.

"I won't participate in anything that's not first tier because we have a first tier campaign," the Kentucky senator told CNN's Wolf Blitzer Monday.

"I'll be taking my campaign directly to New Hampshire and Iowa. I'm not going to be in South Carolina. I think it's a mistake to try to exclude me from the national debate," he added.

Paul also called Fox's decision "a mistake."

"It's a mistake because the thing is we actually have been in the top five or six in most of the recent polls. In fact, last week in a national poll we were just one point out of fourth place. So I think it's a mistake to try to exclude me from the national debate," he told Blitzer.

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