According to the new rules, "any candidate who ranks in the top 10 in polling between August 6 and September 10 will be included," CNN said in a statement.
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"In the event that any candidate is polling in the top 10 in an average of approved national polls released between August 7th and September 10th, we will add those candidates to our top tier debate, even if those candidates did not poll in the top 10 in an average of approved national polls between July 16th and September 10th," the network said. "We have discussed these changes with the Republican National Committee and the Ronald Reagan Presidential Library, and they are fully supportive."
Fiorina's deputy campaign manager, Sarah Isgur Flores, tweeted thanks to the candidate's supporters.
Republican National Committee Chairman Reince Priebus said in a statement that he supports the changes.
"I applaud CNN for recognizing the historic nature of this debate and fully support the network's decision to amend their criteria," he said.
Previously, a spokeswoman for CNN indicated that changing the rules — originally published on May 20 — would violate FEC guidelines.
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"CNN published the criteria for the CNN-Reagan Library debate on May 20th. It will encompass polling data from three weeks prior to the first debate and five weeks following," CNN spokeswoman Barbara Levin told POLITICO. "Federal Election Commission guidelines make it clear that these criteria cannot be changed after they have been published. We believe that our approach is a fair and effective way to deal with the highest number of candidates we have ever encountered."