Unable once again to score a knockout, Sen. Barack Obama is likely to make his new negative tone even more negative -- with a sharp eye on trying to end the Democratic presidential nomination fight after the May 6 primaries in Indiana and North Carolina. Sen. Hillary Rodham Clinton's victory yesterday in Pennsylvania has only accentuated the quandary that Obama faces: Stay negative and he risks undermining the premise of his candidacy. Stay aloof and he underscores Clinton's argument that he will not be able to beat a "Republican attack machine" sure to greet him this summer. Obama campaign manager...