LAS VEGAS, NV - DECEMBER 15:  Republican presidential candidate Donald Trump (C) speaks as Ben Carson (L) and U.S. Sen. Ted Cruz (R-TX) listen as during the CNN Republican presidential debate on December 15, 2015 in Las Vegas, Nevada. This is the last GOP debate of the year, with U.S. Sen. Ted Cruz (R-TX) gaining in the polls in Iowa and other early voting states and Donald Trump rising in national polls.  (Photo by Justin Sullivan/Getty Images)