GOP presidential candidates stand behind their podiums during the CNN Republican presidential debate at the Ronald Reagan Presidential Library and Museum in Simi Valley, Calif., on Sept. 16, 2015. Concern for U.S. democracy amid deep national polarization has prompted the entities supporting 13 presidential libraries dating back to Herbert Hoover to call for a recommitment to the country's bedrock principles, including the rule of law and respecting a diversity of beliefs. (AP Photo/Mark J. Terrill, File)
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