IT IS an old cliche of politics that the best form of defense is attack. This season's Democratic primaries offer an inversion of that thesis: The best form of attack is defense, or at least the appearance of defense. Bill Bradley protests that he is shocked, just shocked, at having to defend himself against the slings and arrows of his adversary. That same adversary, Al Gore, laments that he must defend himself against nasty Mr. Bradley. By defending themselves indignantly and at maximum volume, both candidates are attacking their opponents for attacking them.

The latest offensive defense was mounted by Mr. Gore on Tuesday. "Personal attacks have no place in a campaign," Mr. Gore said, before casting doubt on his rival's professed commitment to reforming campaign finance. But this principled attack on attack tactics was disorientating. In recent weeks, Mr. Gore has insinuated that Mr. Bradley is a quitter, a bad Democrat and perhaps even a player of racial politics. The Bradley health plan, he told a black audience, would harm poor blacks on Medicaid.

Mr. Bradley, to be fair, can be defensively offensive too. He made great play of refusing to respond to Mr. Gore's early barbs, seeking thereby to suggest that his opponent was a dirty fighter. He went so far, in one especially passive-aggressive moment, as to accuse Mr. Gore of "the political opportunism of Newt Gingrich." More recently, Mr. Bradley has turned plain aggressive. He has accused Mr. Gore of using poor people as "political footballs," of being "too timid" on gun control and of failing to stand and fight for health care. He has chosen to illustrate the need for campaign reform by citing the Clinton-Gore abuses of 1996.

The sadness is that both candidates are worthy men, decent in their private lives and serious about public issues. They are both centrist Democrats; they agree on most subjects. But the logic of the campaign forces both men to manufacture differences between them. And so they march into a looking-glass world where defense is attack, where similarities are turned into distinctions and where civility is a tool in the service of ambition.