Though Barack Obama beat Hillary Clinton in the Mississippi primary by an impressive margin of 61 per cent to 37, the win came as no surprise and did little to change the contest for the Democratic nomination. More significant is the growing bitterness of the fight between Mr Obama and Mrs Clinton, and the effect this might have on the eventual nominee’s election prospects.

The feuding worsened after Mississippi when Geraldine Ferraro, a member of the Clinton team, said that Mr Obama would not be where he is but for the fact he is black. Though true – just as it is true that Mrs Clinton is where she is because Bill Clinton is her husband – this was a divisive thing to say of a fellow Democrat. Mrs Clinton distanced herself from the comment, but tepidly, leading the Obama campaign to complain of a double standard. Samantha Power, an Obama adviser, had just been dumped, amid furious protests from the Clinton campaign, for calling Mrs Clinton “a monster”. Yet more proof, says the Obama campaign, that Mrs Clinton is a practitioner of dirty politics, and that their man is better.

Yet whether this theme advances Mr Obama’s prospects is doubtful. For a start, when Mr Obama and his team complain about negative campaigning, they protest too much. Throughout, they have argued that Mrs Clinton, in harking back to the 1990s, stands for division and strife – to be contrasted with his more consensual approach – and that this militates against getting things done. If that is not a “negative” attack, what is? When the Clintons say that Mr Obama started this fight, they have a point.

More important, what is wrong with attacking your opponent? Mrs Clinton is entitled to emphasise Mr Obama’s inexperience, just as Mr Obama is right to question Mrs Clinton’s ability to forge consensus. The correct distinction is not between “positive” and “negative”, but between honest and dishonest. Mrs Clinton has demonstrated a keener taste than Mr Obama for attack by innuendo, by distortion and by deniable surrogate. This “whatever it takes” approach does poison the contest and risks harming the Democratic party, even if it helps her defeat Mr Obama. But Mr Obama is wrong both on the merits and as a matter of tactics to demur from temperate, frontal criticism of his rival.

Until just this week, for instance, he has let Mrs Clinton’s exaggerated claim of superior experience (including “helping bring peace to Northern Ireland”) go virtually uncontested. If he continues to let her get away with such stuff, he deserves to lose.

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