Rep. Ron Paul. Patrick Smith/Getty Images hide caption

itoggle caption Patrick Smith/Getty Images

Former Massachusetts Gov. Mitt Romney speaks to a group of workers at Nationwide Insurance Company, Nov. 23, 2011, in Des Moines. A new poll suggests his religion could be an obstacle in the GOP primary. Steve Pope/AP hide caption

itoggle caption Steve Pope/AP

Hillary Clinton's "inevitability" became less certain after a debate in Philadelphia at Drexel University in October 2007. William Thomas Cain/Getty Images hide caption

itoggle caption William Thomas Cain/Getty Images