Candidates who could challenge the frontrunners
                                
                                
                                    (Newser) -
                                    The American electorate loves a dark horse—the candidate that comes out of nowhere and, against the odds, becomes a serious contender. Barack Obama can claim that distinction, as can Mike Huckabee, whose 2008 bid presaged Tea-Party enthusiasm. But who will it be in 2012? Chris Cillizza has some ideas...