Mitt Romney, whose liberal past and questionable conservative credentials are at the heart of his problems securing the Republican presidential nomination, is being supported by three prior Republican nominees--all of whom lost their bids for the presidency while running as moderates.Sen. John McCain, the 2008 nominee who lost to Barack Obama, endorsed Romney on January 4 heading into the New Hampshire primary.Former Sen. Bob Dole, the 1996 nomineee who lost to Bill Clinton, endorsed Romney in December before the Iowa caucus.Former President George H.W. Bush, who lost to Bill Clinton in 1992, 'unofficially' endorsed Romney in December in remarks to...