Donald Trump, Jeb Bush and Scott Walker will take the stage in Cleveland on August 6 with seven rivals.
Fox News selected the 10 most popular Republicans based on five national polls, excluding Rick Perry and South Carolina Senator Lindsey Graham.
Those two and five other candidates will take part in an earlier debate.
Former Pennsylvania Senator Rick Santorum reacted angrily to his omission.
“The idea that they have left out the runner-up for the 2012 nomination [Rick Santorum], the former four-term governor of Texas [Rick Perry], the governor of Louisiana [Bobby Jindal], the first female Fortune 50 CEO [Carly Fiorina], and the 3-term Senator from South Carolina [Lindsey Graham] due to polling seven months before a single vote is cast is preposterous,” his spokesman said.
In contrast, Rick Perry tweeted that he was looking forward to being on Fox at 5PM for “a serious exchange of ideas and positive solutions to get America back on track”.
The main debate takes place four hours later at 9PM local time.
All eyes will be on Donald Trump, who leads the polls and has made headlines with outspoken remarks about many of his rivals.
One of the Republican frontrunners, Jeb Bush, became embroiled in a row with leading Democratic candidate Hillary Clinton over women’s health funding on August 4.
The former Florida governor was attacked by Democrat Hillary Clinton after he told a conservative Christian audience he wasn’t sure “we need half a billion dollars for women’s health issues”.
Jeb Bush later said he “misspoke” after criticism of his remarks.
Top 10 Republicans Running for White House 2016:
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