2016 Republican presidential candidates Senator Marco Rubio, a Republican from Florida, from left, Ben Carson, retired neurosurgeon, Donald Trump, president and chief executive of Trump Organization Inc., Senator Ted Cruz, a Republican from Texas, and Jeb Bush, former governor of Florida, listen to the U.S. National Anthem during the Republican presidential candidate debate at The Venetian in Las Vegas, Nevada, U.S., on Tuesday, Dec. 15, 2015. With less than two months remaining before the Feb. 1 Iowa caucuses and the Feb. 9 New Hampshire primary, middle-of-the-pack candidates hoping for a late surge in the polls have little choice but to come out swinging in tonight's fifth Republican debate. Photographer: Ruth Fremson/Pool via Bloomberg