LAS VEGAS, NV - DECEMBER 15: Republican presidential candidates Donald Trump (L) and Jeb Bush (R) repond to each other as U.S. Sen. Ted Cruz (R-TX) listens 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)

LAS VEGAS, NV - DECEMBER 15:  Republican presidential candidates Donald Trump (L) and Jeb Bush (R) repond to each other as U.S. Sen. Ted Cruz (R-TX) listens 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)